Penny is the only one of the family whose mutation has permanently scarred her body. She appears as an anthropomorhpic Pygmy Rattlesnake.
Her personal venom is sodium pentathol on steroids, inducing in the subject a complete lack of willpower. She can also secrete a venom that weakens the subject physically.
Penny McCoy is the daughter of another Drusilla DeWitt sibling, Dora, who married Jacob McCoy.