Corr is the Amtrak of capaill—fast, reliable, and hardly ever kills people. The capaill aren't like normal horses, though. Corr has "something in his eyes that the thoroughbreds will never have: something intense and predatory" (20.10). On a regular horse, you might get bucked off or trampled, but you don't have to worry about one of them eating your face.
When it comes to Sean, Corr is downright pleasant. The only time we see him get violent is when Mutt's men start manhandling him. Corr ends up killing one of them, but no one really cares because death by capaill is a daily occurrence. Plus, Sean kind of wants to kill Mutt and his cronies anyway.
Sean and Corr have a lot in common. Sean says, "Corr can hold a thousand things in his heart and reveal only one of them on his face, like he did earlier today. He is so very like me" (43.32). We guess fast, reliable, and hardly ever kills people could be applied to Sean, too. They're brothers from other species's mothers, and when Sean tries to release Corr to the sea at the end of the book, Corr says neigh. Okay, he doesn't say anything, but he does decide to stay with Sean, who by that point is like family to him.