I saw a drawing once here on Elfwood of a horse who had been driven by its master to run until it died. However, the horse never died. Instead, when it was driven beyond consciousness, its spirit snapped and it turned into a horrible undead monster, killed the cruel master and then kept on, running like a maniac and killing everything in its path. I was inspired, and made some other undead creatures. A dragon, a sheep-goat-thingy, and a Jersey devil.