A "Swimming Sharptooth" (Hydrotherosaurus) makes a small appearance in The Land Before Time IV: Journey through the Mists. It lives in the Mysterious Beyond, and possibly lives in a river area, as it appears there in the film.
It appears near the end of The Land Before Time IV: Journey Through the Mists, after Ichy and Dil part ways. After Dil swats away Ichy with her tail, she bumps into the Hydrotherosaurus, who subsequently chases the villainous Deinosuchus away. It is unknown if Dil survived the encounter, or if she was ultimately eaten by the much larger plesiosaur. It is likely that she did not escape the much faster and keen sighted Hydrotherosaurus.
- The Hydrotherosaurus is the fourth Swimming Sharptooth to appear in The Land Before Time franchise.
- This Hydrotherosaurus is the first and so far only one in the franchise.
- It is the first plesiosaur to appear in the franchise.
- It bears a slight resemblance to the famous Loch Ness Monster that is believed to live in Loch Ness, Scotland.
- The Hydrotherosaurus looks similar to a real Elasmosaurus, but Elsie, who is confirmed to be an Elasmosaurus from The Land Before Time V: The Mysterious Island, looks different, due to her having a crest on her head.