This is what would happen if a school shooter had a really cool horse instead of a weapon. 

Don is a relatively popular but put-upon high schooler who has issues with the jock bullies in his school, his parents are having marital issues, and his love life is challenging as well. He's also self involved, self pitying, and aggressive and creepy. His rage pulls a (frankly really cool) giant black horse into reality from a poster in his room, and it starts violently killing anyone who does him wrong.

I was worried that the author would try to paint Don as a totally sympathetic guy, struggling with the nasty boys and the mean girls who aren't willing to be totally at his beck and call! But actually it did a really good job of making his steadily less likeable, and more threatening, boiling the frog-style. 

I didn't think the writing was especially great, and it was a challenge to read an entire book with such unliekable protagonist, but the story was decent and the point it was making was well made. The descriptions of the horse were also undeniably really cool, and easily my favourite pieces of writing in the book.