This question already has an answer here:
A while back, I read a fantasy book about a boy who ends up in another world inhabited by a people called the Sidhe (I think). He got there while visiting an old man at night, but something spooked him and he ran into the garden, where he went through a portal. He was taken in by three witches, until he went his own way. I also seem to remember he could either talk to horses or transform into one. I think the main storyline was that the Sidhe world and the ordinary world were getting too close to each other. Its all very vague.
It was not written by Feist, Asimov, Prachett, Jordan, Tolkien. I read it back in the early 2000's, but I don't know if that was when it was published. I believe it was part of a series, as I seem to remember part two took place in the "normal" world. I also seem to remember the protagonist (boy) was either part Sidhe/part human, or had at least a connection to them.