Alligator

Alligators were a type of predatory Earth reptile.

History
Nero threatened the First Doctor, stating he would build a moat around him and fill it with alligators. However he withdrew from this action when the Doctor inspired him to burn Rome. The Doctor then told his captives the punishment would be reserved for them if he and Vicki were not freed. (DW: The Romans)

During the 22nd century Dalek invasion of the solar system, including Earth, many animals escaped from zoos during the plague. Most of them died but reptiles such as the alligators thrived in the London sewers. An alligator almost ate Susan but was killed by David Campbell. (DW: The Dalek Invasion of Earth)

Similar species
Sundayan 'gators were visually similar to both Earth alligators and turtles (NSA: Wetworld) while the soresox were a species that looked like stone alligators. (NSA: The Doctor Trap)