Poor Mutt. He lives on an island overrun by flesh-eating horses that come out of the water and eat people, yet he's the antagonist of the book. The horses might eat people, but at least they don't pee in their shoes.
Yes, Mutt pees in Sean's boots in a fit of jealousy. See, Mutt's dad likes Sean a lot more than he likes his own son, and Mutt just can't handle Sean's sheer amazingness. Being called "Mutt" doesn't help matters.
The biggest sticking point for Mutt is that Sean is indispensable while Mutt is replaceable. No matter how awesome Mutt tries to be, he can't live up to Sean. Sean tries to quit, and Mr. Malvern won't let him. We have a feeling if Mutt tried to quit, his dad would celebrate.
Mutt's jealousy gets the best of him when he tries to kill Puck during the races. This tactic backfires, and Mutt's horse drags him to a watery grave. Does his dad mourn? We have no idea. The only thing we know is that no one really seems to care that he's gone.