The Doctor and Businessman in “Gridlock”

The Macra used to be the scourge of this galaxy. Gas - they fed off gas, the filthier the better. They built up a small empire, using humans as slaves and mining gas for food.

They don't exactly look like empire builders to me.

Well, that was billions of years ago, billions. They must have devolved down the years, now they're just beasts. But they're still hungry, and my friend's down there.