When heavy rains create a mysterious "new water", Littlefoot sets off to explore the Great Valley. He quickly becomes friends with Mo, a prank-playing dolphin-like creature who can't find his way back to the Big Water. Littlefoot and his pals come to the rescue and prove the value of courage, friendship, and diversity.
The Land Before Time. An image from one of these movies popped up on my Facebook feed, and I couldn't remember the name, so I searched in the comments. But now the only character name I remember is Little Foot. :)