There is my ancient LBT fanfic, Out Of The Shadows. It might have been the first evil Chomper fanfic created, but I'm not sure. I don't recall seeing any others out at the time. The basic premise is that the valley comes under seige by an older Chomper, who wants to...I think take it over or something. I forget. It's been a long time. It gets pretty weird later in the fanfic, and I admit, Chomper's reason for turning bad is pretty rushed and could have been handled better.
Interestingly, the story was the result of a joke. My brother cracked a joke about older Chomper eating his friends, and I took that and used it as the premise of the story.
Does Chomper become good again in that one or does he die in the end?
I did have one theory of how Chomper could go bad. He could have already known that Sharptooth was his uncle or something. However, when he is older, he finds out from the Gang of Five that they killed Sharptooth.
Though he would have known the reason for that, they did something like that with the pretty much good character Harry Osborne in the Spider Man trilogy where he was nice till his father died, seemingly by Spider Man's hand. In the second film, he wants to get Spider Man for it but is still friendly toward Spidey's actual identity Peter Parker.
At the end of the second movie, he finds out, after making a deal with another villain (albeit the deal was kinda forced as he was gonna be dropped to his death otherwise by Doc Ock.) and he agreed to give Doc Ock what he needed for the sun machine in exchange for bringing Spidey to him so that he could avenge his father's death.
However, when he gets Spidey, after unmasking him, he realizes that Spidey is his best friend Peter Parker and can't do it. Later, he is visited by a sort of phantom of his father, the Green Goblin, and, after refusing at first to get revenge, breaks a mirror and finds his father's lair and realizes that his father is the Green Goblin, and also finding weapons that would enable him to get back at Spidey.
In the third film of that trilogy (and, in my opinion, the best movie EVER0, he at first is just grumpy toward Peter, but soon tries to avenge his father's death and yells "You knew this was coming Pete." True, Osborn stops being the New Goblin and even gives his life to save Peter Parker/ Spider Man at the end of the film, but still, there's a type of scenario for a bad Chomper.
Also, it would be interesting in the Sharptooth is Chomper's relative scenario if the Gang of Five, especially Littlefoot, would hold it against Chomper upon learning that he's related to the evil villain of the first movie.
Sharptooth could even have turned out to have only did what he did for food and have been nice to Chomper's parents and even perhaps saved their lives at one point.
(This also is similar to the Spider Man trilogy, in that Norman Osborne is nice to Harry and even tries, perhaps, to get rid of Peter, feeling guilty over having shown a bit more favor toward Peter over his higher IQ than Harry, and promising to rectify inequities. True, Norman had other motives to get rid of Peter, but making it up to Harry may well have been a motive, Norman never tried to hurt Harry. He did, inadvertently, in the Times Square attack, but it wasn't intended.)
