Horda were small predators native to Leela's home planet. They looked like long, armoured fish with large heads. Although Horda could only crawl around slowly, they could strike quickly and ten of them could devour a human's arm in seconds.
The Sevateem had a test using Horda. A criminal would be put on a board above a pit of Hordas. The board would be pulled into the walls with a boulder attached to a rope. The criminal would have to break the rope with a crossbow, stopping the board and saving his life. The Fourth Doctor succeeded at this test. (TV: The Face of Evil)