What are they?
An intelligent, amphibious species that have shapeshifting abilities and some affinity with electricity. Aggressive and ruthless when threatened, they have the technical ability to adapt their environment to optimum thermal level: they like the cold, and find heat intensely painful.
First appearance:
Horror of Fang Rock (1977), voiced by Colin Douglas. The Fourth Doctor (Tom Baker) discovered that a Rutan scout had crash-landed close to Fang Rock lighthouse in the early 20th century. It killed lighthouse keeper Reuben and assumed his identity, also killing six others before Leela (Louise Jameson) blew it up with a makeshift rocket launcher.