Six-legged camel

In an alternate timeline in which the Earth was devastated by a nuclear war started by the experiments of Theodore Cassells, the camels, alongside some humans which derived on the Mutes, evolved on a six-legged species. These were mounted by the Mutes.

A single Mute rider chased the Third Doctor when he arrived in the alternate Earth, but was knocked by a metal piece thrown by Brod. The camel remained with its unconscious rider.

The camels presumably were erased from the timeline when history was restored. (COMIC: A Stitch in Time)

Trivia

 * This is the only creature of all the Doctor Who Franchise to be a non-human "Future Creature" (It's say; a earthling organism evolved due the Earth's changing conditions). a human example of this is the Haemovore, evolved due the harsh environment created by the industrial pollution (The Curse of Fenric (TV story)).