Zeerover

Zeerover were small, purple, penguin-like creatures who liked to heal others. They travelled through space in egg-shaped crafts and were capable of taking over other beings as hosts by touch or proximity. Some Zeerover arrived on Earth by accident in 1348. They found the atmosphere poisonous. The Tenth Doctor helped them leave by replacing their craft's damaged fuel cell. (DWBIT: Plague Panic)