Tirek has been the only "canon" centaur so far, and he clearly differs from the typical model, what with the horns and red skin...

So, My first query is: does he represent a unique species? or is he a half-demon hybrid like some of the fandom thinks?

just because of how I work, i'm inclined to believe the former.

the second question: Do other centaur-type beings exist in this world/your headcanons?

For me, I have a whole family of centaur-like beings, Tirek from the show (i've labeled him a Sagittarius), five taken from actual mythology, and one from World of Warcraft

Sagittarius
Big
Red skin
Horns

Centaur
Alicorn-sized (?)
Tannish-brown skin

Onocentaur
Pony-sized
Tannish-brown skin
Looks like half Dwarf, half Donkey

Icthyocentaur
Alicorn-sized(?)
Green
Aquatic (short legs)

Magnataur
Big
Grey-brown
half-mammoth

Alcetaur
deer-sized
light brown
half deer

Urmahlullu
alicorn-sized
tan-gold fur
can be mistaken for a monkey-faced winged lion

In my headcanon, Centaurs are culturally similar to the Dothraki from "Game of Thrones," and live on the steppes between the badlands and the San Palomino desert.

3777303 I consider him to be a type of demon. His brother looks nothing like a centaur, even though they're the same species. So they have to be something else.

In my own headcanon and story you only get centaurs from human male and female pony copulation.
Like so

They grow to the same size as normal humans.

3777368 That makes a lot of sense for a lot of reasons.

The Dothraki are a horse-centered culture (ironically making them the closest thing to an MLP fanbase in the GoT-verse).

the centaur myth may have originated from a non-equestrian culture meeting men on horseback

3777672
If they ever appeared in the show, I'd like them to be led by this guy:

http://www.deviantart.com/art/Elder-Centaur-108585809

and have him be voiced by Clancy Brown.

3777685 I like my pointy-ear version, personally, but that's pretty cool looking!

https://www.artstation.com/artist/jeehyung

I found this, and it seemed appropriate to leave it here. I love the feathers.
The human torso may not be quite large enough - I don't think she can quite reach the ground.
The arms may also need to bulk up to be reasonably strong.
But most excellent aesthetically.

5447668 agreed.

and you're right. the torso should probably be less elfin and more muscular. if the lower body's a draft horse, the upper body needs to be more like this

(pic is not mine, this depicts a humanized version of Peachy Keen, the daughter of Cheerilee and Big Macintosh, drawn and made by Lopoddity)