Noob Heavies
Pootis Noobius Noobius
A wonder of nature, prey to the classes with lower health.
These guys are idiots. These are the people often seen running straight into the fray with their bare fists. They are the ones commonly seen to punch each other in the spawn room. These guys are hard to kill, not because they are tough, but because they stay in the spawn room "tending" to the rest of the team. They have two known subspecies:

Chargers:
Pootis Noobius Taurus
The Chargers run straight into battle, like a bull blissfully unaware that it has no horns. They fight only with their fists, unless accompanied by medics.

Nesters:
Pootis Noobius Spawncampius
Nesters hide either in the spawn room, where they punch their Teammates and block the supply cabinet, or the intel room, where they do similar behaviors. They hide in either rooms for their own protection, and only fight when they are attcked by intel grabbers.

When attacked by Noob Heavies, the best way not to be defeated embarassingly is to move out of punching distance and spam the FIRE button until they die. They dont usually have any non-stock weapons, as one might think they are almost always new players.

This creature is a combination of Medic and Heavy. They are a force to be reckoned with depending on the kind of species either is. Noob Heavies with any Medic is a waste of ability, as the Heavy will continue to assault the team even if the Medic dies, or may be the first to die anyways. But paired with a good Heavy and a decent Medic, they are volatile, even to sentry nests, and possibly even the rare and deadly Sentry Hive.
They are difficult to kill, and very unpredictable, as they can make do with any weapon, with a few small exceptions. If you wish to kill them, you should try to snipe them from a distance, starting with the Medic, then the surprised Heavy host. Their weakspot is behind and above them, unless flanked by other members of the team. Being a symbiotic pair, it can include any species of both Medic and Heavy.

Pro Heavy
Pootis Rex
These guys are deadly. Forget the Medic. Forget the entire other team! Either get out of his way, or get killed. If your in the intel room, prepare yourself. If your his target, prepare to die, no escape, no help. He will get you. Not sentry Nest or Hive can stop him. Even if his death is inevitable, its too late, youre already dead.
They choose only the biggest and baddest weapons to bring down their prey. If you are caught in their crossfire, the best way to take them down is to lure them into a group of Pyros. They are not easily caught off guard and are the top priority for your hit-list.

Brash Heavy
Pootis Narcissus
Brash Heavies arent always the best players, but they sure want to look that way. They dress themselves in the most expensive Mann co. Merchandise, which they often obtain by sucking their parents' credit cards dry. ( Depending on whether they live with them or not.) They love to look better and stand out, which they certainly do the best. Theyre that one guy on the team that looks nothing like everyone else. They always call MEDIC! first. They are relatively dangerous, but are very cocky and have very heavy (Haha) egos. The best way to kill them is to make their target look defenseless(i.e. Hiding sentries, other players, etc.) and swarm them.

Troll Heavies:
Pootis Trollus
These guys are tricksters, and will do anything if it will p**s you off. They are largely unpredictable, because, like artists, they have different styles.They are, however, easy to kill if you catch them off guard. They are similar to the Brash Heavy except they show off to troll, and not to make themselves look better. Just corner and spam the M1 button until they die, but prepare for raging, as they cannot take what they dish. And will vote to kick you for no reason, and continue trolling if they succeed.

Hoovy
Pootis Pootis
Hoovies are the more docile of the Heavy family. They enjoy more leisurely pursuits such as group taunting with others of the herd, and eating delicious sandviches of course. You may compare their temperament to a neutered bull. A bull is still a bull nonetheless, so refrain from angering them. If you are a Hoovy from the opposite team, you can join their herd if you crouch, holding a sandvich, and walk slowly towards them. Drop your sandvich as a peace offering. They will most likely present you with the opportunity to group taunt with them. Doing so will gain their trust, and that is the best way not to anger them.

Angler Heavy
Pootis Espionagus
Anglers are a rarity. I myself used the angling method for a time. They are rare because their method requires a lot of patience, and TF2 is a place where patience is looked down upon. Their method is simple: they place their sandwich on the ground, hide until an enemy comes and thinks they scored a free lunch, and then the Heavy fills them with lead. These guys most commonly carry a tomislav to ensure that their prey can't hear them. This tactic can be useful, but only in a few cases. If you stumble upon a seemingly lost sandvich just sitting on the ground, don't pick it up. Just walk away. Or even better, kill the Heavy that put it there, that's one less Heavy for your team to fight.

Credit to The Gaming Genesect for the discovery of this species
Pootbird
Pootis Aviarus
Status: Somewhat Rare; Exclusive to trade servers
Also known as the Pootis Bird, the Pootbird likes to nest in crowded trade servers, using Sandviches to lure it's favorite prey, Medics. They are like mixing a Hoovy with an Angler Heavy. After hunting, it will return to the flock, sharing Sandviches with those who didn't catch anything. There has been a recent decline in pouplation as Medics become increasingly scarce, and this species may soon be prone to exctinction.

Bear Heavy
Pootis Ursus
Status: Endangered
These majestic creatures once roamed freely on servers, but have since lowered in population, some even being forced into trade servers, where they scavenge for medkits and stray classes. They are very territorial, and encounters with other Bear Heavies tend to result in fatalities. They often carry a Dalokohs bar or Steak Sandvich, but are oddly not very fond of normal Sandviches, but will eat them if necessary. They are always Bear-themed of course, with a wolf hat and those bear-claw things. If you encounter one, keep a distance, as they usually only use their fists for fighting, unless a minigun is necessary.