Macra

The Macra were a gigantic crustacoid race who surivived on unclean gases which were poisonous to Humans, but would apparently happily take them when they could. (DW: The Macra Terror, Gridlock)

Physical appearance
As stated above, the Macra were massive crustaceans. They had large eyestalks and gigantic claws, and were a dark, mucky brown. In all other respects, they were extenally identical to normal Earth crabs. (DW: The Macra Terror, Gridlock)

History
The Macra were an ancient race with a large empire, "the scourge of the galaxy" as the Doctor put it. However, after many millions of years they degenerated into feral beasts. (DW: Gridlock)

On one planet colonised by Humans, they used the Humans as slaves to mine the gases that were vital for their existance. They were defeated when the Doctor effected an uprising on the planet and created an explosion in the control centre on the planet, killing the Macra in charge and apparently dooming the species on the planet. (DW: The Macra Terror)

On New Earth, a colony of several million Macra survived in the depths of the Motorway in New New York, feeding on the poisonous fumes (which supposedly caused one Human's head to inflate to "40 feet across" according to Thomas Kincade Brannigan) and stray people who came down in their cars to the "Fast Lane". They were apparently again doomed when the Doctor opened the roof of the Motorway, allowing the cars to espace to the Upper City, depriving the Macra of food and gases. (DW: Gridlock)