After trying to find food for the dislocated members of the Great Valley, in order to keep the group together, Littlefoot and the others are marooned on an island, where the ocean surrounding it prevents the homesick Littlefoot and gang from leaving. Their problem is eventually solved when Elsie appears, offering the kids a ride to the mainland. They accept, and they travel safely back to the other members of the Great Valley, who have found enough food to last them until they could finally return to the Great Valley. Elsie claims that Littlefoot has inherited his "good looks" from his grandfather. Cera, because of her tendency to experience motion sickness, objects to further "rides" on Elsie's back. She is the only plesiosaur that has a name.
Two Elasmosaurus appear in the opening of The Land Before Time IX: Journey to Big Water. They both appear similar to Elsie because they have a similar short horn on their heads. It is possible that one of these individuals could be Elsie herself.
She is similar to the Hydrotherosaurus as both saved the characters from the villian.