Yep. Slytherin for him too.
I had originally thought Hufflepuff, because he is a pretty laidback, sweet, kind horse.
On the other hand, he is whip-smart, strongly dislikes hard work, is not terribly loyal (ie, doesn't make a lot of friends with other horses). The Harry Potter Wikia also tells me that Slytherins have a highly developed sense of self-preservation, which also fits him beautifully. He is not ambitious in the human sense but he is in the horse sense: he's always got an angle on getting what he wants, whether that's figuring out how to unlatch the stall door, sneaking more food, or planting his feet on the trail.
(In terms of further classification, I myself am a fairly classic Ravenclaw.)
Expanding this outward:
Eventers are in Gryffindor.
Dressage riders are in Ravenclaw.
Jumpers are in Slytherin.
Hunters are in either Slytherin or Hufflepuff, depending on which stereotype you want to go with.
Ranch horses are in Hufflepuff.
Fox hunters are in Gryffindor.
Anyone else care to place either their personal horse or their discipline within a Hogwarts House?