Bernie Kosar from I Am Number Four has the outward appearance of a beagle who is John's pet. However, he shows himself to be much, much more. He is a guardian pet from Lorien, called a chimera. His original name was Hadley, but when he arrives at John's school wearing a collar with the name Bernie Kosar, it sort of stuck. HE is able to change his form into anything organic. At some points he's a lizard, and at other times transforms into a huge battle dragon.
