Laluni

The Lalunis were a species of metamorphic beings who were after the secret of time travel for years.

Trahlen and Kyle were two Lalunis who were trapped in an alien zoo to try and get time energy that was leaking out around the planet. Trahlen turned into a crocodile, and became nicknamed "Time Crocodile" for his ability to travel through time. Kyle was a tortoise.

Later, they both became humanoids, but with a crocodile and tortoise heads. They were defeated by the Doctor. A mysterious alien Time Agent took them away. (NOTDWU: The Time Crocodile)