Per legend, all hippogriffs are by definition the result of griffin-equine mating. That would make him MLP's first officially mixed-race character.

They've mentioned mules on the show before, so I suppose technically not first.

But I do mean that they could just as easily just make hippogriffs a large race in the grand scheme, maybe even their own homeland somewhere. Then they just coincidentally happen to appear like the spawn of pony and griffon banging…

Or yeah, the more mythologicaly accurate method of having them as half-breeds. So possibly few in number since there are probably few pony/griffon couples who are Equestria citizens.