Snake

The true form of the Mara was snake-like. (DW: Kinda)

Snakes were a variety of reptiles of the suborder Serpentes on Earth.

In the Arizona desert, the Tenth 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 Mind Robber)

Giant purple Ice Snakes lived on Centuria. (DWBIT: Enemy Mine)

The Church once fought Lava Snakes. (DW: The Time of Angels)

A giant species of snake called the Oroborus was able to consume time. (K9TV: Oroborus)