Hm...
at 1, it tries to run over you for about 6 damage.
at 2-4, the thing kicks you for about 3 damage.
at 5-10, it runs off, scared.
at 11-15, it'll edge away from you, nervously.
at 16-18, it'll let you ride it once.
at 19-20, it'll pretty much just accept you as its owner.

So, roll to get horse, I guess.