Good things about Horatio:

• Looks good with Horatio Cane mode.
  
• Tons of manpower for all manpower stuff - including new boarding mods.
  
• Has food. (I heard that is a rather attractive trait about people.)
  
• Scales well into endgame

Bad things about Horatio:

• Scales slowly and needs specific minor/major civs assimilated to be competitive with other factions.
  
• Is ugly without the horatio cane mod, heroes look a like, can get confusing
  
• Does not have that much food really, unfallen have more.
  
• Ship slots are versatile but mostly meh
  
• Expensive ships

I personally really don't like horatio that much, while the thought is nice, the execution can be frustrating and problematic, and even if played properly, the affinity just does not provide sufficient bonus and is too circumstantial when compared to those of other races.
Now let's set the proper mood for reading this guide, go and watch the CSI:Miami intro.

Traits:

• + 1 pop on planets (weaker than cravers)
  
• + 1 Z'vali citizens which you will need for asimilation
  
• + 100% hero recovery rate (not very noticeable) so that your heroes can get fooled again.
  
• + 25% ship cost, which is not gamebreaking but hurtful
  
• Starting tech for arid and Minor diplomacy

Affinity Gene Hunter:
Convert N units of pop into horatio pop bonus. Cost scales up with each assimilation by 2 in Fast speed.
Best races to convert are the harderst to get - cravers, riftborn and vodyani. Riftborn will probably not have enough numbers and you can only get vodyni pop by converting or succesfully invading their system and cravers ofc should kill you easily.
For full set of bonuses visit the horatio wikia wiki page[endless-space-2.wikia.com], I will provide some info here, but the details are already explained there. Not a big fan of copy paste guide writing.
It is advisable to convert ASAP, as the leftover pop should be inferior to horatio pop and thus occupy a pop slot suboptimally.
It is nice, but you need to stack a lot of pops to make it work, which are not always available (hardly ever).
If you seek aditional interesting statistics, I suggest you refer to ITER on G2G forums[www.games2gether.com], which has some (ir)relevant data of all sorts.
Starting planet:
Arid - this sucks cause it does not scale well with any T1 upgrades (it's NOT temperate and NOT fertile losing you 66,66...% of potential bonuses from the early FIS improvements.)
Starting hero:
Some Horatio Overseer guy...
Has neat scouting slots but just 2.
Skills not too impressive, something related to food and luxuries, all real nice, but until super endgame not interesting.
Starting gov:
Dictatorship - comfy if you don't need multiple laws, enables you to push for the endgame laws in party of your choosing since turn 10. But eventually I suggest you switch to republic. Basically not being a republic is a drawback in my opinon under most circumstances.
Ecologists is very comfortable starting party, but I would not stay in it too long. -50% FIDSI penalty on planets you do not have tech for is too hurtful. However this enables you to colonize almost any planet. Horatio also has special set of laws tailored to assimilation gameplay, issue is, it relies in having something to assimilate, which you might not have. If you want to have something to assimilate, probably go militaristic and pew pew stuff and benefit from eco later, for which you need 2 parties and more than 1 law slots, thus you will still need to change gov eventually.

Pop collection bonus:
Unlike Lumeris, Horatio actually have a very neat 50 pop collection bonus, it enblaes influence production proportional to food, enabling you to run expensive laws with ease. Which is another reason why you should switch govs from dictatorship.

You are the endgame monster, but then so is basically every other race, just without to prerequisite to assimilate other races.
So technically you start behind and fight to catch up. And unless you collect real good set of races into your genom, you will probably be average at best. However, this is not a deal breaker, as you have some neat faction specific techs which provide you with a bit of extra manpower.
Anything beyond that sucks.
So in the early try to make best of your free colonization range, but bear in mind the eco fidsi penalty applies to the food income and consequently the outpost growth.
RNG good luxuries and you should be good.
Hug all those per-pop bonuses and industry upgrades, move pop around.
Try to form an early game fleet and invade away some pop to grow and assimilate.

The point at which you shine is utilizing T2 per pop luxuries and the food to industry conversion. This however, is so far in the endgame on current patch, that you basically don't shine and should die sooner.
Of course having every other race in the game assimilated makes you insanely OP, however this is under most circumstances close to impossible.

Some general generic pointers, just watch the video.
These steps are heavily RNG reliant on your surroundings, however, thanks to horatio being ecologist, you can always find a place to expand to. Otherwise go with Bear Grylls advice:

Turn 1:

• Assing hero to fleet and search curiosities.
  
• Spam probes, your scout ship can have all slots probes, give it a retrofit and repeat.
  
• Create fleet from the probing ship to isolate hero, give hero probes and send him somewhere do curiosities.
  
• Send colony ship to hopefully scouted good system with strategics or luxuries. Or just send it blindly, it can colonize whatever, you are ecologist!
  
• Either do the dust/science upgrade or colonize a planet, preferably one with a luxury or strategic resource, if there is one, personally recommend getting the +15 science first, and of course, don't forget about colony ships and expansion.
  
• Get T1 strat tech either titanium, or whichever you have available. Xenoindustrial upgrade is crap for you unless your next planet is fertile/temperate.
  
• Enable the Cram exam law, your pop is content either way and you can make a tiny bit more science this way

T2:

• Send the probing ship to probe too, probably in the other way
  
• Befriend a minor faction, if you find any, preferably purely by 10 influence praises and scanning, watch out warmonger factions engage once they have ships and are below 50 relationship. Try to get them through quests, if they suck, just invade.
  
• Tech towards system upgrades with your luxury, if you have any, otherwise ship tech, getting to T3 in miltary
  
• Get the industry upgrade quest and work towards it, others are meh, I know it is not "cool" but it's what you need to stay in game and actually matter.

T5

• Assign hero to system, unless there is more to scan close by. If you do have a fertile, give him
  
• food per fertile skill.

T10

• Change government, but remember you can no longer colonize all. Probably militaristic and declare war on someone for approval boost.
  
• Focus on getting those T1 luxury upgrades on your systems, you could have 3 systems by now, also focus on research to compensate, where your luxury is lacking - for instance if you have bluecap mold, you can probably skip on most +science buildings and their research and leave it for later.
  
• Start get fleet, wreck stuff, you have surprisingly decent industry outpot early on - 2 tools for movement speed + weapons with T2 beams work nicely for early to mid game and are not that hard to get.
  
• Don't forget about conerting MP to tanks.

T20

10 Command Point cap is nice. Focus on that while you are have those T2 beams, right? Don't forget about those minor factions, you can again, go militaristic, freely wardec and invade them.

T30
Good time to think about those T3 lasers and extra slots on T1 ships.
If you are not dead, just keep doing what you were doing. Could have defeated one player by now, try to avoid draining too much pop, so you can assimilate it faster.
T40
I don't really know anymore, from here on it's really not something you can predict and depends on your interaction with other playes. Just watch the video or something...
Here is the video, it then links to the following video.

Since this is not so unique across the factions, perhaps it would be smarter and less cluttery to use another guide for it?
You bet ya!
Click here for combat stuff, fittings, setups and combat advice.

As suggested earlier in the guide you should follow the industry path, which is most competitive for the extra industry output it gets you.
At the end however, you can take the Religious turn, which lands you with two lvl 10 heroes. Which are both horatio, but at least one is seeker so he is somewhat consequential, otherwise the quest can be safely ignored after achieving the early goals as the endgame goals are not too influential.

Horatio sucks.
Optimal use would be rushing people on small maps, maps with high count of minor factions so you can utilize your affinity early, otherwise it is just meh and pain to play and manage.