|Information in this article is non-canon|
This is a movie where Marty McFly Jr. recreates the old time machine and goes back in time to the time of the dinosaurs. The story is here: https://www.fanfiction.net/s/11740450/1/Back-to-the-Land-Before-Time
After Chomper and Littlefoot have an argument, Chomper goes back in time and ends up stranded. Angry at Littlefoot, he eventually grows up to be the villain, Sharptooth, killing Littlefoot's mother and everythiing. Realizing the damage they have done, Littlefoot and Marty Jr. set out to make things right.