Originally, the Yends looked humanoid, 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 only for a few minutes at a time. (PROSE: The Sons of the Crab)
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, 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 species. 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 buildings 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 realised that he was something different, they brought him to their leader, Fomal. After explaining their situation, the Doctor agreed to take one hundred Yend embryos to another planet. While he was leaving, the mutants finally broke into the facility, spreading the changing disease further. Though the Doctor escaped, all the embryos died the moment he entered the Time Vortex. (PROSE: The Sons of the Crab)