A Complete Catalogue?

I had tamed a cat before, one. I had found a village. It quickly died though, but thats not the problem. After I tamed the cat I got the achievement "Best Friends Forever". I left the village and continued exploring until I found a horse. I tamed the horse and the achievement "A Complete Catalogue" popped up. I moved my mouse over it and it said "Tame All Cat Variants". A horse is a cat variant??

?!?