Noob Engineer
Engie Noobus
Status: Somewhat Common
These fascinating creatures are fascinating, common, and unlikely to win a "smartest Texan" award. Often sporting a Gibus and a shiny new Mercenary Badge, these common critters can be found anywhere and everywhere. They mainly carry stock weapons, but usually posses at least one non-stock wrench. They have a very loose description behavior-wise, as many are overly trustful and rarely spycheck, while others are as paranoid as Luigi on LSD, constantly with their back to a wall and an eye on their comrades. These are two subspecies of Engie Noobus, and can be called the Dodo Engie and the Loner Engie.
Dodo Engie
Engie Noobus Dodoformis
This subspecies consists of the newest batch of TF2 players. They strut about with their Mercenary badge and stock weapons while asking why everyone is conga dancing and standing idle. They don't know how to turn their sentries, so they just leave them in the open, free for a wandering Spy to prey upon. Speaking, of Spies, they don't see them as a threat, as they think their sentry makes them akin to God. They aren't usually problems, aside from catching people off guard with randomly placed sentries, and can be easily dealt with.
Loner Engineer
Engie Noobus Major
Unlike their cousin the Dodo Engie, these engineers are a lot less trustful. Like, a lot. They tend to have a non-stock weapon and a Gibus/Pyrovision combo. They always assume their trusted comrades are all dirty spies, waiting to stab in the butt the first chance they get. These guys are often getting over the trauma of their first net sapping, and their distrust is somewhat reasonable. But they take it to the extreme. Back to dispenser, constantly hitting sentry, staying as far away from the fight as possible extreme. They won't let anyone near their builds, even other Engineers. This separates them from the Turtle Engie, as the turtles are much friendlier.

Turtle Engineer
Engie Tortlus
Status: Very Common
This is a rather docile creature, sitting in one spot all day without moving. They are friendly, patient, and overall a good defender. It may be hard for them to establish themselves, but once they do, they are a solid wall of defense. They come in many shapes and sizes, but you only need to listen to these steps to idenify a turtle.
1. They always build a dispenser first.
This is why they have trouble settling into their job. While most would build a sentry first, they wait before building weaponry. This helps them make sure they can support the new sentry, as well as build faster.
2. They stay in one basic area.
Although they may move a bit to make teleporters, they generally stay in one area, either in the intel room as defense, or hidden in the enemy base, as a sort of checkpoint. This helps keep the flow of battle more equal, and every good team has a turtle.
3. They trust you.
There are a few exceptions every now and then, but they usually only spycheck if they know a spy has been lurking around lately. They are very open to help, and are quick to help other Engineers.

Parasite
Engie Parasiticus
Status: Rare
This deadly fellow is thankfully very rare. Parasite Engies build sentry nests, like most others, but unlike the others, they nest in the ENEMY base. They usually hide in small, hidden areas, building as much as they can. After they finish, they invite other Parasites to help expand. If left unchecked, they can become a serious problem. Parasites reside almost exclusively at ctf_parasite, so cappers need not beware, but for intel snatchers, it can become increasingly difficult to operate. Their weapons prevent foes from exiting, and spies with intel from entering, and it takes a lot of Spies and Medvies to wipe them out, so beware the Parasite kiddies.

Credit to Bmc01 for helping name the species
Assault Engineer
Engie Combatus
Status: Somewhat Uncommon
This little fella can be a bit of a nuisance for Scouts and Medics as they, as their name implies, love to fight at the front lines, rather than tend to the base. They commonly carry a mini-sentry and dispenser around with them, and aren't too hard for a Medvy to take on, but for most everybody else, even the Spy, they are a thorn in the side. They carry around big guns for making a necessary escape, such as The Widowmaker, or for easy repair, like that gun-hand thing nobody uses. They will usually plop themselves at a vital area, such as a bridge or door, and will sit completely still until their victim stumbles by mistakenly. Some really skilled individuals can survive on metal from guns alone, but these few rarely appear. Unlike many other species, it doesn't specialize in just one gamemode. It is a jack-of-all-trades, and a master of most, and goes by many names like the Battle Engie or Combat Engie. It currently has two unnamed subspecies.

Credit to David Conagher for the discovery of this species
Leaping Devil Engineer
Engie Assassinitus
Status: Extinct Critically Endangered
Assassinitus is ridiculously rare. I have yet to see one myself, and I search eagerly for new specimens. Once widespread in the Badwater area, the residents deemed them too dangerous, and they were slowly hunted to near extinction. They changed tides in many wars by using a unique skill used by few others in the Engie genus: Sentry Jumping. These mad devils fearlessly launch themselves through the air, crushing their surprised prey beneath their feet. Although once believed to be extinct, there are some have survived, retreating to trade servers and Tc_Hydro, which have had dozens of sightings.

Credit to David Conagher for the discovery of this species
Domestic Engineer
Engie Godditus
Status: Critically Endangered
Godditus is an extremely rare species to find. So few exist that it is rumored to be exctinct. They are not known to be very aggressive, and can be tamed. Upon finding one and gaining it's trust you will have found a lifelong companion. They warm up quickly to new players, and will support their teammates as best as possible. They are so loyal, that some will even take a bullet for a dear friend, and will give their lives to protect those they care about.

Trollgineer
Engie Trollus
Status: Somewhat Common
Every class has its trolls, but few are like the Trollgineer. Loyal to his team, as most of the Troll species are, this fellow is skilled in the art of trolling, so much in fact, that it has become an apex troll, alongside the Troll Spy. Like a spider, these crafty guys will set traps. A Level 3 Dispenser is something no Spy can resist, especially not Troll Spies. It will skillfully hide a sentry elsewhere before waiting for it's prey to fall for the trap. This doesn't always work, as the Troll Spy preys upon the Trollgineer in a sort of symbiotic relationship, limiting the troll population for everybody to keep it (somewhat) peaceful. The Trollgineer is often easy to offend, so don't insult it in the chat box.
Brash Engineer
Engie Narcissus
Status: Overpopulous
Unlike the Trollgineer, the Brash Engineer is simply a jerk. They dress in the most flamboyant of hats, donning fancy crests to mimic the Alpha Engie, which is usually what they hope to become. And unfortunately, this works. It has been discovered that the natural predator of Engineer nests is Medvies, and recent decrease in Medvy population has sparked an overpopulation of Engie Narcissus. Programs have been set to introduce a subspecies of Medvy to keep the population down, but Medics are scarcely found these days. If you find a Brash Engie, do the server a favor and kill it.

Credit to David Conagher for the Discovery of this species
The Enlightened
Engie Monkriartos
Status: Critically Endangered
Monkriartos is commonly associated to be the Patriarch in a pack of Engies. More often than not, they have observed every map, from Badland to Coldfront, Hightower to Rottenburg, and so on. This makes them a formidable strategist in nearly every situation, and can change the entire flow of battle. Each of this species has it's own style, and rarely give away their brilliant strategic secrets. When not fighting, they train other Engineers in their arts, thus earning it's title of The Enlightened, and teaching them not only how to fight, but how to live.

Credit to David Conagher for the discovery of this species
Plains Engineer
Engie Gottamus
Status: Very Rare; Almost Exlusive to trade servers
The Plains Engie is one of the friendliest creatures you will ever meet, wandering everywhere in search of the Hoovies and Pootbirds it shares it's home with. These simple nomads wander about servers, lugging dispensers on their backs as gifts to new friends. These creatures were nearly hunted to extinction by invasive Spies and Snipers, but has begun to make a comeback since the "Love and War" update, leaping from barely a hundred, to over a thousand. Upon encountering an opposite team member, it will crouch in front of them and place a dispenser in a gesture of friendship. But even though they are coming back from a massive population drop, they still have one enemy: Competitive Players. They are often seen as worthless by Competitive Players, and are targeted for cheap kills. The only place where they can get peace is in trade servers, amongst conga dancers and Sandvich nommers.

For the discovery and contribution of more than five total species residing in the Engie genus, I, Mr.Pixl of Atlanta, Georgia present David Conagher with the TF2 Biology Scientific Award. Congratulations, and keep up the excellent work.

Credit to flyingspider54 for the discovery of this species
The Foolgie
Engie Stupidus
Status: N/A
This is an unusual creature in that it behaves much like an Assault Engie by using Mini-Sentries at it's disposal. Unlike the Engie Combatus it stays on the defensive side, waiting silently in the intel room. It can be regarded as stupid by some, but I have a theory. I believe that it uses this tactic because the Mini-Sentry is, well, mini! It most likely uses the small size of the Sentry to make it less noticable, as well as harder to hit. It may also just be very, very stupid. But thats just a theory. A TF2 theory. (Insert shameless parody here).

Credit to multiple people, this was my most suggested
Ninjaneer
Engie Espionagus
Status: Unknown; Not enough research
Whoo. This one was requested a LOT. By the way, the rest was written by msuic, with some minor alterations.
This creature is a rare one, usually running the Widowmaker along with the Eureka effect. A rare breed indeed, this species seems to have caught the eye of some well-known youtubers, who have adopted the strategy. This species can commonly be found on attack/defend maps. Placing a hidden teleporter exit, but no entrance so it makes no noise, and has no eye-catching particle effects. They will often teleport to this hidden exit in order to build a recently common annoyance: Spawntrapping Sentries! This Sentry is build for the sole purpose of distracting the enemy team. There are a few who change up their tactics in order to become more specialized. They can be found mostly in Frontier and Gorge.

Basic Sentry Nest
Engie Rectin
Status: Very Common
The Sentry Nest is a complex hierarchical system of both Sentries and Dispensers, which are tended to by the following:
Sentry Guard
E. Rectin Asentry
These fellows, as stated by their name, guard the Sentries, checking for Spies, repairing damage, etc., and they are often stronger than other Engineers, being the backbone of the entire nest. Their job can be partially replaced by a Pyro, allowing them the freedom of placing teleporters.
Dispenser Guards
E. Rectin Adispenser
If the Sentry Guard is the backbone, then the Dispenser Guard is the bloodstream keeping the nest healthy and functional. They guard the Dispensers, which are often placed in clutches to allow mass production. Their job can be partially replaced by a Sentry, or better yet, a Heavy, allowing them the freedom of placing Teleporters.
King
E. Rectin Regalis
This patriarch leads the other Engineers, telling them where to place Sentries and Dispensers, or just being a figurehead in particular. They are often well dressed and tend to be mimicked by Brash Engies who hope to gain dominance.
Tele-Builder
E. Rectin Ateleporter
Upon temporarily abandoning it's post, any Engie can become a Tele-Builder. Their task is simple, build a teleporter in the enemy base, and return to their post.

Aqua Nests
On some very rare occasions, nests will be built at the bottom of ctf_well. Dispensers are always built first, preventing the inhabitants from ever having to leave their aquatic homes. Not much is known about this tactic, other than it works.

Credit to David Conagher for the discovery of this species
The Drifter
Engie Movertus
Status: Somewhat Rare
Movertus is a somewhat rare species, but one would think it is common, due to other species choosing to mimic it. This species looks for a well protected spot to place it's Sentry, and then continues on it's nomadic journey. They use this tactic to keep enemy focus on the Sentry instead of the Drifter itself. It tends to avoid other Engineers, which makes it more stealthy and more likely to produce builds with long lifespans, lasting as long as entire matches. They are known to like small maps and confined spaces.

Thank you, community, for suggesting this
The Relaxoneer
Engie Relaxo
Status: Variating
The Relaxoneer is a calm and deceptively busy creature. At first glance, the Relaxoneer may seem akin to the sloth in it's behavior, but these creatures are amazingly adapted to the multitude of environments it occupies. There are more subspecies than any Engineer: Engie Relaxo MvM, Engie Relaxo Bakerus, Engie Relaxo Moveritus. There may even be more that we have yet to discover.
The MvM Relaxoneer
Engie Relaxo MvM
Status: Somewhat Common
This creature is one of the most common Engie to be found on any MvM server. They are quick builders, and can even endure Sentry Busters if a Medvy is present on the team. After finishing all of their builds, they get a well-deserved rest as they enjoy the carnage. Sadly, this subspecies is at a decline due to the decreasing Medvy population.
The Bread Farmer
Engie Relaxo Bakerus
Status: Critically Endangered
This subspecies broke off from the MvM Relaxoneer at the Love and War update, and the population exploded. This species supposedly went extinct when the Gun Mettle update and bread was made to stop appearing. It has been recently discovered that bread DOES in fact appear, but far less frequently than before. Still, the Bread Farmer Relaxoneer still clings to life in the very farthest corners of the game. Conservation efforts need to be put into play.
The Nomad Relaxoneer
Engie Relaxo Movertus
Status: Somewhat Endangered
Although not very, this subspecies is endangered. They never were common, being in competition with the Nomad Engie and The Drifter. This species builds a Sentry and relaxes for a small bit until, for reasons unknown, it moves to a different spot and repeats the process. Some theorize that it follows it's prey to new feeding grounds, but this remains untested to this day.

(Thanks to https://steamhost.cn/steamcommunity_com/id/dacloony for the discovery of this fascinating species, keep up the good work!)
Useless/Avian Engineer
Engie Lazarus
Status: Uncommon

This species of Engineer is the result of an odd fusion beteen a leaping devil and a relaxo engie, and may be a subspecies of either, but it's lineage is unclear.
This species of Engie most commonly sets up a sentry and dispenser near a hard to reach spot.
The Engie will always have a wrangler, so be on the lookout,because you might have found one. The Engie will rocket itself into an Out-of-map area, and if they fail, that is why they have a dispenser, to heal any lost health. Upon success the engie will plop down its toolbox and sit, for hours. To tell if one is in the area, just look for a mass group of reds (or blus depending on what team the UE is on) staring up at the engie, most commonly shooting at it, with the occasional rocket/stickyjumper attempting a rather useless attack. Another sign is if you see reds(again, blus too depending the UE's team) yeling in chat about someone outside the map. If you see one, dont let the other team know (if they dont already) about the matter, let 'em figure it out themselves.

Dispenser Jockey
Engie Praedictum
Status: Somewhat Uncommon
This species of Engineer is unusual in that, like the Noob Engie, the behavior of this particular species is rather unpredictable. Some members of the species are friendly, while others are aggressive towards the enemy team. Each individual does share a common behavior in that they all like to perch on top of their dispensers. How this behavior spawned is up to speculation. Some suggest that this behavior was used to make spies more vulnerable to other Engies, and others suggest that the Engie uses his perch to survey the landscape and watch for enemies other than Spies specifically. Many in the Species are friendly, and will nest with the enemy team to make a web of traversable dispensers. The less friendly ones will use their sentries as a perch as well, but tend to prefer being alone. Some may classify the unfriendly individuals as a separate subspecies, but I don't see enough difference to classify them as separtate.