Some TMNT stuff really isn't for little kids.
Leatherhead is a bipedal mutant alligator. In his first appearance, he is wearing torn orange pants and a metal waistband. Later, when he is living in New York City during "Urban Legends", he is seen wearing an improvisedly-sewn sleeveless shirt, a scarf, a pair of blue jeans with a hole for his massive tail, and white gauze wrapped on his arms. In "The Trial of Krang", he is again wearing his previous orange outfit.
Leatherhead is very intelligent, articulate, and mannerable to a certain extent, until he is provoked, which can cause him to become aggressive and violent.
Leatherhead first mutated on Burnow Island during the 18th century, and is at least that old. His mutation came when a nearby pirate ship unloaded some of its cargo into the water, which included canisters of mutagen. Leatherhead was named by the island's former indigenous inhabitants who only saw him in brief glimpses, and nicknamed him "Leatherhead of the Lagoon." When Leatherhead met Honeycutt and the turtles, he claimed he had lived a solitary life for over two centuries, but this proved to be a lie—he had been General Krang's enslaved pet for most of that time, and only escaped when the Foot Clan invaded the island.
When Krang's Technodrome terraformed Burnow Island, it finally became uninhabitable for earth life, forcing Leatherhead to seek shelter with Professor Honeycutt, where he also became acquainted with Harold Lillja by remote communication. Leatherhead shared his mutagen stash with Honeycutt to heal Donatello's broken body and revive the Utroms from stasis, in exchange for being allowed to use Honeycutt's teleportation device to leave Burnow Island and travel to New York City.
But Leatherhead had a secret agenda—he wanted revenge against Krang and all the Utroms for his centuries of imprisonment and torture. To this effect, he murdered a handful of Utroms in stasis, then dismembered and nearly killed Honeycutt who walked in on him in the act. When his actions were revealed, he fought the Turtles and nearly killed Michelangelo in his efforts to reach New York. Upon being transported there, he was overwhelmed and had to flee.
He appeared again in Urban Legends, when Old Hob tried to recruit him for the Mutanimals. However, their wildly varying opinions on mutants led to a conflict.
The Neutrinos also sought out Leatherhead as a surprise character witness against Krang during The Trial of Krang, since Leatherhead could personally attest to Krang's cruelty. But when King Zenter decreed that Krang would serve a life sentence on Burnow Island, Leatherhead was enraged and took justice into his own hands by eating Krang alive. He then escaped into the portal, back to Earth. Having achieved his revenge, Leatherhead soon departed Burnow Island for New York, and pledged to protect "natural" creatures from mutants, after saving a seal from a mutated monster.
The Turtles encountered Leatherhead again in a museum, where he had lost two fingers to a M.O.U.S.E.R. Leatherhead reiterated his belief that mutants should not exist, but when Raphael confronted him about it, the alligator began to hallucinate that Krang was taunting him. He lashed out at the Turtles and nearly injured Michelangelo before fleeing back into the sewers, still hearing his nemesis' voice. This was later revealed to have a medical basis, as Krang began to regrow parasitically in the mutant's abdomen.