Stingray

Alien Stingrays were creatures that lived in swarms of billions and due to their sheer mass were able to open wormholes in space by moving at fast speeds. They were named Alien Stingrays by UNIT due to superficial resemblance to the stingrays that live in Earth's seas.

Biology
The Stingrays had an exoskeleton of metal to enable them safe travel through the Wormholes they created. Their digestive system allowed them to eat anything including metal which was used to make their exoskeleton ,once digested They were seemingly unintelligent and incapable of speech. They acted on the instinct to devour and move on. They moved from planet to planet by creating wormholes. They created these by swarming around the planet many times so their sheer mass would rip space creating a wormhole they then travelled through. Their exoskeleton could protect them agaisnt bullets but not explosives as shown when UNIT managed to dispatch three of their members using a mounted rocket launcher when ordinary rifles had no effect.

History
A swarm of Alien Stingrays opened a wormhole that led them to the planet San Helios. There they swarmed the planet and devoured everything, turning it into a barren desert. They then began making a second wormhole with which to leave the planet. A number of the stingrays were trapped in the hull of the Tritovore's ship. Lying dormant due to its low temperature, but when Christina de Souza went to retrieve the ship's crystal nucleus, her body heat raised the temperature just enough to revive the alien stingrays, which then tore through the ship and devoured the two Trivore occupants. Malcolm Taylor managed to close off their latest wormhole that lead to Earth. However, three stingrays made it through. Conventional firearms were useless against them, but UNIT's mounted missile launchers proved more effective. The ones trapped on San Helios could easily make another worm hole but the Doctor now guides all their wormholes onto uninhabited planets, where they could feed on natural structures.