He has sometimes reared his head. One notable instance was in the 2nd movie:
Chomper attacks Cera
Admittedly, this Chomper seems to go away after they tell him off. At least, for this film.
When we first see Chomper in film V, however, he is even MORE dangerous than the one that bit Cera in II.
However, after that moment, we never see this Chomper again. He isn't
like this in the TV Series nor, unless I'm wrong, in Journey of the Brave either.
Do you think Littlefoot has driven that Chomper out of Chomper or do you think he's stilll there, somewhere and has yet to be driven out of him?