The Horse in Harry's Room by Syd Hoff
Harry has a horse that nobody else can see. Every night, he rides him in circles and jumps over his bed without even hitting his head on the ceiling. Harry loves his horse, so much that he even gives it the freedom to leave and be where he can run and kick and nibble.
The Horse in Harry's Room is a great story about imagination and friendship. This is a great story for a young reader who loves animals and thinks outside the box.