Some TMNT stuff really isn't for little kids.
Slash is an experiment of StockGen Laboratories's that is freed due to April O'Neil's diversion to grab their Turtle Tracker. He punches out one of General Krang's Rock Soldiers and escapes the lab into the night. Before leaving, he takes a miniature palm tree from the tank.
Prior to his escape, Slash, then called Specimen 6, was a snapping turtle that was injected with mutagen from Old Hob's DNA to turn him into another mutant to hunt down the Turtles and Splinter. As Specimen 6's size and strength increased, his mind became unstable and he turned savage which led him to be contained.
After his escape from Stockgen's, Specimen 6 lurked around New York as a mindless monster damaging property, scaring, and injuring people. The beast was called "Slash" for his slashing rampage. Slash's latest attacks were striking at Michelangelo's pizza delivery friend Woody Dirkins with minor injuries and leaving claw marks on April's van.
Slash spotted the Turtles and decided to follow them after ripping off the black shirt of a thug the Turtles have just beaten and turning it into a mask similar to the Turtles. He followed them all the way to an abandoned church which the Turtles take as their new home. Slash then starts to attack them in the basement of the bunker shelter of the church. His strength and speed were too much for the Turtles even when they temporarily subdue him. When Slash was about to kill an unconscious Michelangelo, Leonardo threw a shuriken at him to get his attention. Leo tried to reason with Slash that they were experimented on at Stockgen's too, but the reckless beast jumped him and was accidentally impaled by one of Leo's katana. Slash pulled out Leo's katana and ripped off his mask then fell into the sewer system of the basement as he gave in to his wound.
Slash is revealed to have survived and is washed ashore. He is awakened by Old Hob, who somehow survived being shot by Baxter Stockman, Hob tells Slash that they should team up together to get revenge on Stockman and everyone else at Stockgen. Hob and Slash are co-founders of Hob's new team, the Mutanimals.
It seems more and more that it's become far too easy for us to sacrifice the good in order to confront the bad. The others may not see this as Mikey and I do, but they've never suffered Hun's venom the way we have. You've invited a serpent into the garden, Old Hob... ...and that is something I simply cannot abide by.
My whole life, I've felt like someone else's property. First I was Stockman's lab experiment, then later, despite his best intentions, Hob's plaything. After all the amazing changes I've gone through, that feeling of inadequacy has never left me and I think it's past time I venture out on my own and finally learn what it is to be my own master. But before I go, I want you to know, Mikey, you have always been—and will always be—my hero. Farewell, my friend.