Macra

The Macra were a gigantic crustacean race that fed on unclean gases which were poisonous to humans. They would consume humans when possible.

Eventually, the Macra fell into evolutionary decline, reverting to mindless creatures in the depths of New New York.

Biology
There were two known types of Macra, the original sapient species and the later predators of New Earth. Both fed upon poisonous gases and organic lifeforms when available.

Sapient species
The sapient species of Macra were larger than humans, able to hold a human in one claw. They were stocky in build, with large eye-stalks and large claws, and had a second, smaller set of claws beneath the mouth. They were a dark, murky brown colour. They were also covered in hairs. (TV: The Macra Terror)

Predator species
The predator species of Macra were much larger in size, several hundred times larger than a human. They were more instinctive and less logical than the sapient Macra they descended from, having fallen into evolutionary decline.

They had gigantic claws, large enough to easily hold an entire New Earth car, which were proportionally much larger than the rest of their bodies. They had glowing eyes on thin stalks and six spindly legs. These Macra were predators who hunted by sensing movement, and showed no signs of intelligence. They also had distinctive and terrifying roars. (TV: Gridlock)

Empire
The Macra were an ancient race with a large empire - "the scourge of this galaxy," as the Tenth Doctor put it. (TV: Gridlock) When their home planet was colonised by humans, the Macra took control of the colony and hypnotised the humans into mining gases for them. (TV: The Macra Terror)

During the Eternal War, a Macra killed Gith, causing him to regenerate. Another Macra later beheaded him on Lerna. (PROSE: The Multi-Faceted War)

Their control over the humans on their homeworld came to an end when they were defeated by the Second Doctor, who effected an uprising on the planet and destroyed the control centre with an explosion, killing the Macra in charge and apparently dooming the Macra on the planet. (TV: The Macra Terror)

At some point before 2157, the Guild of Adjudicators dealt with the Macra. (PROSE: Lucifer Rising)

A sapient Macra visited Beachelguese with a Silurian and a Sea Devil. (COMIC: Surfshock)

Devolution
After many billions of years, they evolved into feral beasts. On New Earth, a colony of Macra survived and multiplied in the depths of the Motorway in New New York, feeding on the poisonous fumes and stray people who came down in their cars to the "Fast Lane". They were apparently again doomed when by 5,000,000,053, the Tenth Doctor opened the roof of the Motorway, allowing the cars to escape to the Upper City, depriving the Macra of food and gases. (TV: Gridlock) The Macra preyed on Scorpionkind, though Senator Hame believed Scorpionkind to be worse. (AUDIO: The Cats of New Cairo)

Following the opening of the Motorway, the Macra got a "new career" cleaning up the city's pollution. However, the Cybermen also used the Macra to begin disposing of non-humans. (COMIC: In-Flight Entertainment)

River Song had met the Macra, or at least, had heard of them. (PROSE: River of Time)

Other references
In the video game Happy Deathday, played by Izzy Sinclair on the Time-Space Visualiser, a Macra was among a host of "every single enemy" that the Doctor had ever defeated, who were assembled by the Beige Guardian and pitted against the Doctor's first eight incarnations. (COMIC: Happy Deathday)

Behind the scenes

 * The Doctor labelled the Macra as the "Scourge of this Galaxy" in Gridlock, a title he also gave to Baltazar in The Infinite Quest. However, they are from different galaxies.
 * In The Macra Terror, the Macra seen to be in charge through the porthole in the last episode is white rather than brown (a scene for which the sole Macra prop had to be repainted).
 * Approximately forty years passed between the Macra's first appearance on Doctor Who and their second. This is the third longest such interval between appearances for any character or creature in the franchise. The Great Intelligence is in the lead with forty-four years, then Alpha Centauri with 43 years. Axos had the longest wait to return in all media, with a thirty-nine-year gap.
 * According to Doctor Who: Creatures and Demons, a non-narrative source, on New Earth, the colony of feral Macra that preyed on the New Earth Cars in the Motorway escaped from the New New York Zoo when the power broke down and made their way into the Motorway.
 * Macra also feature in the Decide Your Destiny book, Claws of the Macra, where a group of the feral kind seen in Gridlock infest a gas refinery on Earth.
 * In Gridlock, the Doctor refers to the Macra "devolving". However, in real world terms, this is simply evolution: adaptation of a creature to its given environment. The modern concept of evolution is not progress towards anything; rather, it's an incredibly dynamic process. Each organism traces the path of its evolution only in the direction of the greatest chances for its survival (and for the fulfilment of needs) within a given context; the "more evolved" are not those closer to a higher-order ideal, but those best suited to their own circumstances. Evolution thus cannot operate in reverse, however we might classify the degree to which a creature is "primitive", compared to ourselves.