Tetraps, natives of the planet Tetrapyriarbus, appear as large fur covered bat-like creatures with large leathery vestigial wings hanging under their hairy arms. Four eyes, positioned in the front, back, and either side of their head allow them the ability to see in four different directions at once. Their large ears suggest an exceptional sense of hearing. Like bats, Tetraps feed upon the blood plasma of other living creatures. Tetraps have the ability secrete a powerful venom. Before feeding, they inject this toxin in the neck of a victim with their tongues, leaving the subject incapacitated and in a tense trance-like state. Tetraps have rudimentary intelligence and with training are capable of using technology.
The Tetraps are driven by their desire for sustenance in the form of blood plasma. They have virtually driven every other creature on Tetrapyriarbus into extinction by their feeding habits. So desperate are they to obtain a new food supply that they are willing to do nearly anything in exchange for blood.
The Doctor encountered the Tetraps on the planet Lakertya, where the Rani was using them as henchmen. But, as the Rani later discovered, the Tetraps are not an entirely submissive race... When the Rani's plan failed, the Tetraps kidnapped her to their home planet where her genius could be exploited in their quest to obtain more blood plasma.