Found: Book like Lloyd Alexander

I remember reading a book that I believe had a horse on the cover. A scene I particularly remember involved escaping from a prison. A boy and his friend were in adjacent cells, or so he thought. He told this girl to help his friend from the next cell escape, but once they are out of the prison he is dismayed to find she got the wrong person. It was, I believe, medieval in setting.

That does sound a lot like a scene from The Book of Three, but you're saying it's only similar to that?

Yes, but let me check.

This is exactly what happens in The Book of Three (some editions of which have a horse on the cover). The Prydain books were inspired by The Mabinogion, so possibly Alexander was channelling an earlier story when he wrote that scene (I'm not familiar enough with The Mabinogion to know).

Okay, so that must be it.