Stingray

The Swarm, (TV: Planet of the Dead) a group made up of creatures known as "Stingrays" (PROSE: The Time Lord Letters) were creatures that devoured everything on a planet and could travel through wormholes to new ones. They were named "alien stingrays" by UNIT due to their superficial resemblance to the stingrays that lived in Earth's seas.

Biology
Stingrays had a wide head with little eyes and gills, a long tail and fins down their side. They were the size of a small car, but were still capable of flight at speeds of at least 300 miles per hour. They were extreme omnivores, capable of eating meat, plants, rock or metal. Any metal they ate would be extruded into their exoskeleton, making them bullet proof, though not immune to explosives. Stingrays would also go into hibernation when cold.

Their metal exoskeleton allowed them to produce large amounts of static energy when they travelled in groups, which was capable of creating wormholes. After eating everything on the planet, the whole swarm would start to circle the planet. The sheer mass and speed of billions of individuals would rip open the fabric of space, creating a wormhole, allowing them to travel from planet to planet and continue this life cycle. Their metal exoskeleton would allow them to survive the passage.

They appeared not to possess any real sentience and acted on primal instinct alone. When talking with Captain Erisa Magambo, the Tenth Doctor stated that what they do isn't their fault and is just their instinct. (TV: Planet of the Dead)

History
Circa 2009, the Stingrays opened a wormhole to the planet San Helios, where they ate everything within a year, leaving only barren deserts. They then began making a second wormhole with which to leave the planet. When a Tritovore ship came to the planet to trade, it ran into the swarm. In a manner similar to birds clogging the engines of a plane, the Stingrays clogged the vents of the ship, causing it to crash from ten miles in the air. At least two Stingrays were involved in the crash and were put into hibernation by the cold.

After devouring San Helios and turning it into a desert, the Stingrays began generating a new wormhole to move on. The wormhole eventually opened up into Earth, accidentally pulling the 200 to San Helios. Though they saw the Stingrays from a distance, they were unable to identify what they were without the help of the Tritovore's probe. Later, when Christina de Souza went to retrieve the crystal nucleus from the Tritovore ship, her body heat raised the temperature just enough to revive the hibernating Stingrays. Though Christina managed to kill one with the ship's security grid, the second devoured the two surviving Tritovores.

The Tenth Doctor was able to return the 200 just before the Stingrays made it to the wormhole and Malcolm Taylor managed to close the wormhole after them. However, three Stingrays made it through. Conventional firearms were useless against them, but UNIT's mounted missile launchers proved more effective.

After the threat to Earth was over, the Doctor admitted to Captain Erisa Magambo that the Stingrays would simply start again and generate a new wormhole. However, the Doctor acknowledged that their actions weren't their fault, being instinct and stated an intention to try to "nudge" the Stingrays wormholes onto uninhabited planets where they could feast without ever harming anyone. (TV: Planet of the Dead)