Ah, a boy and his dog. I mean, horse.
The Pet by Charles L Grant is a lovely story about a boy who wants to be a vet. He has posters of animals on his walls, stuffed animals and other fake creatures in his room. His parents don't approve.
In the midst of his teenage angst (girls, school, bullies, and parents suck) he needs someone to talk to. So he begins talking to a poster of a black stallion on his wall. One day he notices that the stallion is fading from the picture and eventually disappears completely. Later he realizes that his friendship has freed the horse.
The horse has a mission. Take care of all those people who are bothering his friend. A serial killer, a football player, a science teacher, and almost girlfriend and parents are taken out. Will he send the stallion back to the poster in time?
At its heart, this is a "be careful what you wish for" tale. More importantly, what do you do when you get what you want and you don't want it after all? If you accidentally make a wish and want to immediately take it back?
All in all, a good read.