The Tetraps are a bat-like species from the planet Tetrapyriarbus. When the Doctor encountered them on the planet Lakertya they were being used by the Rani to help her control the natives.
The Tetraps are humanoid and have wings that help them jump from large heights. They have four eyes around their head, but it can take them a few moments to focus on an object.They are strong, and are able cause paralysis by injecting a victim with their tongue. On Lakertya Mel was paralysed, but the Rani had an antidote.
One of Beyus's duties on Lakertya was to look after the Tetraps, who sleep upside down, and feed them a blood-like liquid.