A two headed, eight eyed mutant horse
Mutant Horses are creatures similar to Slow Mutants. They have been affected and disfigured by air pollution and ither outside forces. But unlike Slow Mutants, they were once normal horses, as opposed to humans. Roland encountered Mutant Horses in the Pricetown area. In "The Talisman", the main character sees a 2-headed horse behind a tent.