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 to a six-legged species. These were mounted by the Mutes.

In 5000, 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)