Yend

The Yends were a humanoid species which unfortunately were mutated into a race of involuntary shapeshifters.

Biology
Originally, the Yends looked Near-Human, with thin bodies and large foreheads. After the mutations, they were constantly in flux and had no set shape. With great force of will, they could stay in one form, but this was only possible for a few minutes. (DWA: The Sons of the Crab)

History
The Yend were originally a humanoid species on the planet Wengrol. Much of their science was focused on biology and they started to genetically modify themselves perfecting organs and adding new parts. Unfortunately, at this time the star Mortain arrived in their solar system, bathing the system in radiation. This radiation, combined with their genetic modifications, caused uncontrollable mutations in the entire planet. They were unable to escape, since all the other planets of the system were also bathed in radiation. The scientists of the planet built massive lead builidings in the hope that they could find a cure.

Many centuries later, the First Doctor came to Wengrol during his travels. He was caught by a group of scientists, who believed he was a Yend who had managed to stabilise his form. Once they realized that he was something different, they brought him to their leader, Fomal. After explaining their position, the Doctor agreed to take 100 Yend embryos to a another planet. While leaving, the mutants finally broke into the facility, spreading the changing disease further. Though the Doctor was able to escape, all the embryos died the moment he reached the Time Vortex. (DWA: The Sons of the Crab)