An alligator, as distinct to a crocodile, was a predatory Earth reptile.
History
Nero threatened the First Doctor, stating he would build a moat around him and fill it with alligators. However he changed his mind when the Doctor inspired him to burn Rome. The Doctor told Nero's guards the punishment would be reserved for them if he and Vicki were not freed. (TV: The Romans)
During the 22nd century Dalek invasion of 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. (TV: The Dalek Invasion of Earth)
Similar species
Sundayan 'gators were visually similar to both Earth alligators and turtles (PROSE: Wetworld) while the soresox were a species that looked like stone alligators. (PROSE: The Doctor Trap)
