Skaarg is a criminal from Dimension X in the 1987-1996 animated TV-series episode Convicts from Dimension X. He and his friend Dementor were convicted criminals in the Dimension X Penitentiary, which they unsuccessfully attempted to escape from.
However, a tear in the dimensional fabric opened in their cell, allowing them to escape to Earth. The two wasted no time in acquiring weapons in order to conquer the city, which attracted the attention of April O'Neil and the Turtles. Skaarg and Dementor then made their way to the Turtles' lair, where they used Donatello's portal to bring the Dregma brothers to Earth as well. Having done so, they left a bomb on the portal to destroy it and kill the nearby Turtles.
Skaarg, Dementor and the Dregma brothers then made their way to the National Guard armory, where they constructed a flying tank covered in weapons. They were attacked by the Turtles once again, who lured them to the specific location that they came through, where a portal opened up to pull them back to Dimension X. Once there, the guards re-apprehended them.