Tongue Monster

The Unknown Corrupted Gem is a corrupted Gem that first appeared in "Kindergarten Kid".

Appearance
The Unknown Corrupted Gem is a bipedal, avian-esque Gem, with two long dark green legs which lead into an orb-shaped and teal colored body. On either side of its head are a set of three small eyes with white scelera and lime-green irises. Its has a large mouth with fang-like teeth and a set of three gray tongues it often undulates when calling out while bobbing up and down. Its gem is located on the center of its forehead and is a mixture of blue-green colors that are different on each facet.

Abilities

 * High Speed: It can run at extremely high speeds, making it immensely difficult to capture, even when trying to grab onto it such as when Amethyst latched onto it using her whip.
 * Enhanced Strength: It is capable of towing gems like Amethyst and Peridot behind it while still running at maximum speed, and was easily able to lunge onto Steven and attack him using its talons.
 * Intelligence: It was capable of outsmarting Peridot and Steven's plans, and even inadvertantly caused Peridot to get crushed by a massive boulder through its curiosity.

Kindergarten Kid
The Unknown Corrupted Gem was the main antagonistic force in the episode. It was left in the Beta Kindergarten after Jasper had been corrupted and poofed as a remnant of her army which was being rounded up by the Crystal Gems. When Pearl, Garnet, and Amethyst were incapable of capturing it, Peridot and Steven went out to hunt it down.

Trivia

 * It is revealed in the episode, "Ocean Gem" that the Worm Monster, along with other monsters, are corrupted Gems.
 * The worm monster's gem reappears in the online game, "Heap of Trouble".
 * Small blue heads of this creature appear in "Gem Bound".