The Judoon are intergalactic enforcers clad in tough, black armor and look like a cross between a stocky human and a rhinoceros.
Employed by the Shadow Proclamation to police the universe, they have limited intelligence but plenty of determination and focus. Described by the Doctor as “interplanetary thugs," these mercenaries first appeared in the episode "Smith and Jones," when the Tenth Doctor met Dr. Martha Jones.
They have a slavish obedience towards rules and when carrying out a law enforcement mission they’re single-minded to the point of brutality, dishing out "justice"—usually execution—instantly and without compunction.
During early encounters with the Doctor they implied he was not their enemy and their pursuit of the Time Lord on the moon was basically a case of mistaken identity. However, they did join the alliance of the Doctor’s old enemies in "The Pandorica Opens."