Penelope Rex might finally have bitten off more than she can chew — a pet sabretooth tiger! Most kids have dogs and cats and fish and guinea pigs. Penelope Rex has Mittens. Penelope wasn't sure she even wanted a pet when Mittens came into her life. But now that he's here, she is determined to make the best of it. Pets, especially ones that are six hundred pounds with six-inch-long teeth, can cause very big, very messy problems.