The P'tarr were barrel-shaped and had four clawed feet. They had tails with bony spikes on the end.
At one point, partially Cyber-converted humans landed on the P'tarr planet, hoping to exploit it. The P'tarr, with the help of the Fourth Doctor and Leela, defeated them, and the Doctor extracted a promise from Cain that he and his men would help to keep the planet secret. (PROSE: One Bad Apple)