Slash is often the antithesis of the Teenage Mutant Ninja Turtles, being very much like them, but on the opposite side of the law. In most continuities, he is a mutated pet turtle, just as they are.
1987 series continuity
In the original cartoon series, Slash is the pet turtle of Bebop. Bebop and Rocksteady were tired of having to do chores around the Technodrome, so they mutated Slash to get him to work instead. Slash was of extremely limited intelligence, and prone to fits of rage, especially when his "binky" (a plastic palm tree) was lost or in danger. Later in the series, Slash's intelligence was upgraded by an alien race. In this series, he was voiced by Pat Fraley.
Slash in the Archie Comics series Teenage Mutant Ninja Turtles Adventures was stylized a great deal closer to his action figure counterpart, but with a few slight differences (such as skin color). As opposed to being a mutant turtle like other incarnations, Slash is instead from an alien world, full of palm trees called Palmadise. Invaders, however, came and cut down all of his beloved trees, enraging the turtle. He later came across an exiled Krang and agreed to help the warlord return to Earth. Although often an enemy of the Turtles, he eventually befriended them and their other friends, the Mutanimals. Slash sacrificed himself by piloting a spacecraft into the sun, taking the major villains Null and Maligna with him.
Slash was an experiment in Baxter Stockman's StockGen Labs, along with the four turtles that would later become the Teenage Mutant Ninja Turtles, and the rat Splinter. He was an unstable savage, attacking people throughout New York City as well as eventually coming across the Turtles. After his defeat at the Turtles' hands, he befriended the antagonistic cat mutant Old Hob. Hob and Slash sometimes ally with the Turtles and Splinter when the two factions share a mutual goal, or if they need favors traded. After being injected with a vial of Splinter's blood (which contained a more stable version of mutagen), Slash's mind became unclouded and he was able to think and speak clearly for the first time ever.
2012 series continuity
In the Nickelodeon CGI animated series, Slash started life off as Spike, a tortoise kept by Raphael as a pet. After becoming accidentally mutated, Spike remembered all of the times that Raphael vented to Spike about how much his brothers bothered him, and thus sought to defeat them in Raph's honor. Raphael, however, still cares a great deal about the other Turtles, no matter how much they irritate him. Slash later returned as the partner of Dimension X despot Newtralizer, hunting down The Kraang. Newtralizer was unfortunately willing to sacrifice human lives in order to reach their goals, which Slash was not willing to do. With the help of the Turtles and Casey Jones, Slash stopped Newtralizer's plans. In this series, he is voiced by Corey Feldman, who previously voiced Donatello in the first and third Teenage Mutant Ninja Turtles films.
In Video Games
Slash appears as a mini-boss in Teenage Mutant Ninja Turtles III: The Manhattan Project. Leaping off of the Bebop's pickup truck, he battles the Turtles on a dilapidated bridge.
Slash appears again in the Prehistoric Turtlesaurus level of the Super NES version of Teenage Mutant Ninja Turtles IV: Turtles in Time, replacing the arcade version's Cement Man boss.
Slash appears as a boss character in the Nintendo 3DS game based on the 2014 Teenage Mutant Ninja Turtles film.