I don't have a clue how long I have owned an old paperback copy of The Pet by the late author Charles L. Grant but I have had it quite a long time, and I'm going to take a wild guess I'd probably bought it at a thrift store.
The Pet (pub. 1986) centers around Donald "Don" Boyd, a seventeen-year-old who would rather be spending his time with animals than doing typical teenage stuff, such as playing football. His father (Norm) is the principal at the local Ashford, New Jersey high school and he's always giving Don slack about not having a girlfriend and questing his choice of wanting to attend a veterinarian school. To make matters even worse, Norm is frustrated with the teachers' contract negotiations and takes out all of his anger on Don. Add in some bulling from a few football jocks, I guess you can say Don's life isn't exactly perfect.
Late one autumn night, Don runs into the serial the killer "The Howler" and barely survives the encounter. The Howler is later found dead in a park. The authorities come to the conclusion that Don killed The Howler in self-defense, which leads to everyone calling the boy a hero. The truth is Don is not even close to being a hero.
It seems a black horse, which looks oddly similar to the horse in a poster on Don's bedroom wall, was the one that saved him that night. And this is no normal horse! The horse is supernatural and will attack anyone who threatens Don's well being.