The Stag (also known as the Deer or Freak Deer) was a character introduced in "No One Can Hear You". The Stag rampaged through the Candy Kingdom, licking its residents as it went along. Finn and Jake soon arrived to stop it, but were beaten. Jake sustained a serious head injury, causing him to go insane (until Finn bashes his head against a wall a few times). Stag broke both of Finn's legs twice, and then kicked him into a pole, sending Finn into a coma. Stag's saliva is orange, and it appears to have an adhesive property, which he uses to capture Candy People, drag them into the sewers, and then stick them to the walls in a drainage room because he "wanted their sugar." His two front hooves are actually gloves which conceal two human-like hands with opposable thumbs. He also has the ability to walk upright, which he uses while fighting Finn and Jake.
Finn and Jake eventually come to rescue the citizens in the sewer where they confront him. Stag is knocked unconscious by two falling cinder blocks that were attached to Jake's feet, and then he is thrown into a whirlpool. Assuming Stag has similar properties to real life deer, he is a male due to his antlers.
He makes a short appearance in the episode "Sons of Mars". Magic Man turns him into a telescope so that he can take a closer look at a "falling star" (which turned out to be Grob Gob Glob Grod), and has not changed him back since.