Snake

Snakes were a species a reptile.

In the Arizona desert, the Doctor and Donna encountered an alien which had mutated into a giant rattlesnake. (DWBIT: Sting of the Serpent)

Styre's experiments on Sarah Jane Smith caused her to hallucinate a snake on her arm. (DW: The Sontaran Experiment)

A type of giant snake was native to Metebelis III at some point in its history. (DW: The Green Death)

The Mara took the form of a giant snake. (DW: Kinda, Snakedance)

According to Greek mythology, Medusa had snakes for hair. (DW: The The Mind Robber)