Slash was Bebop's ordinary pet turtle first appearing in the episode "Slash: The Evil Turtle from Dimension X", that was mutated by Rocksteady to do chores Shredder had assigned that they didn't want to do. In this incarnation, his obsession with palm trees was in particularly focused on the plastic tree in his bowl before the mutation which he called his "Binky."
When Bebop accidentally dropped it down an airduct he went berserk, stealing Shredder's Shaolin swords and chasing them though the Technodrome before going to Earth due to some quick thinking on Bebop's part. There, Slash ran into corrupt resourceful business man Fenton Q. Hackenbush that wanted to build condos in the sewers, and after being thwarted by the TMNT, he decided to tarnish the Turtles' reputations. He met Slash and decided to send him to smash the Freedom Bell, a patriotic landmark in the city, which would give the Turtles a bad name in exchange for giving him palm trees. The Turtles eventually found Slash, and the evil turtle was able to take on all four of them himself. However, Hackenbush's plan backfired, as he refused to give Slash his reward, because he had come to him while he was live on Channel 6 News, and in response, Slash stole his trash rocket.
Through a series of unfortunate circumstances, he was eventually sent into space aboard the trash rocket, but the trash contained a small plastic palm tree, so he was content. He returned through the Trans-Dimensional Portal portal back to the Technodrome, where he dumped his cargo of waste on Shredder and Krang out of contempt. Slash returned in Donatello Trashes Slash where he ran into an alien race of super-intelligent beings that hooked Slash up to a machine that gave him genius level intelligence (gaining a British accent as prominent evidence).
He returned to Earth with all sorts of fancy weaponry and equipment of his own design, and planning to turn everyone on the planet into turtles, so he would be the number one supreme turtle. Slash managed to turn Vernon and Burne into turtles and was planning to use the Channel 6 satellite dish to turn the entire city into turtles.
Donatello finally tricked Slash by telling him that if he just turned the Teenage Mutant Ninja Turtles into humans, then Slash would be the only mutated turtle. When Slash tried to blast Don, the heroic turtle threw Vernon and Burne into the way to revert them to normal. As Slash fought the turtles, he fell off the roof and onto the street, which reverted him back to his low-level (even more so) intelligence. The Ninja Turtles led him back to the trash rocket and sent him back to space.
Slash also appeared in "Night of the Rogues" having somehow returned to Earth. He along with other TMNT villains (Leatherhead, Rat King, Tempestra, Scumbug, Antrax, and Chrome Dome) is enlisted by the Shredder to help defeat the Ninja Turtles. Like the others, he eventually left when his payment was destroyed.