Tetraps were somewhat like a combination between a human, bat and boar. They were stocky creatures covered in brown fur. They had sharp, clawed hands and vestigial wing-flaps around their arms. Tetraps had four eyes: one in the front, one on each side, and another on the back. They had poor vision and needed to be facing the same way in order to properly focus. They also had four ears, two facing forward and two facing back. Tetraps had a forked tongue which could inject a paralysing poison. They fed on blood plasma, which they called amsalp and slept upside down like bats. (PROSE: Time and the Rani).
The Tetraps were employed by the Rani in her experiment with a time brain on Lakertya. They would bully the native Lakertyans and enforce the Rani's will. The time brain would then use a strange-matter asteroid to change the universe more to the Rani's liking. When the Tetraps learned that the experiment would kill them (and the native Lakertyans), they kidnapped the Rani to bring her back to Tetrapyriarbus, planning to use her to conquer the problems with their source of plasma. (TV: Time and the Rani)
After bringing the Rani to Tetrapyriarbus, they put her on trial. They agreed to postpone the death sentence if she could help them with a shortage of food supply. With the Rani's instruction they kidnapped a man and woman from Earth and two aliens to use for breeding purposes in providing food. But the Rani was able to work with the four "guinea pigs" to escape in a Tretrap ship. After returning the humans to Earth the Rani took one of the alien survivors to another world where they could work to retrieve her TARDIS and punish Urak. (AUDIO: The Rani Reaps the Whirlwind)