Snail

A snail was a type of gastropod mollusc (TV: The Ark in Space) found on Earth and other planets.

Earth
In 2012, pollution in the Vltava River caused snails to mutate into flesh-eaters. The Fourth Doctor solved this problem by getting Drakos to eat them. (PROSE: The Dragons of Prague)

Other planets
Mal Oreille was home to a species of snail, one of which was observed by Bernice Summerfield. (PROSE: Biology Lesson on Mal Oreille)

Snails were also known on Gallifrey. The Doctor's cousins of the House of Lungbarrow gave him the nickname "Snail" because, unlike Loomed Gallifreyans, he had a bellybutton. (PROSE: Lungbarrow, Divided Loyalties, Apocrypha Bipedium, AUDIO: The One Doctor)