What did the horse find common in humans and Yahoos?

The horse found similarities in their appearances.

He agreed; and I went on by assuring him, that the ship was made by creatures like myself; who, in all the countries I had travelled, as well as in my own, were the only governing rational animals; and that upon my arrival hither, I was as much astonished to see the Houyhnhnms act like rational beings, as he, or his friends, could be, in finding some marks of reason in a creature he was pleased to call a Yahoo; to which I owned my resemblance in every part, but could not account for their degenerate and brutal nature.