A "Swimming Sharptooth" (Hydrotherosaurus) makes a small appearance in The Land Before Time IV: Journey through the Mists.
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.
- The Hydrotherosaurus is the fourth Swimming Sharptooth to appear in The Land Before Time franchise.
- This Hydrotherosaurus is the only one in the franchise.
- It is the first plesiosaur in the franchise.
- It bears a slight resemblance to the famous Loch Ness Monster that is believed to live in Loch Ness, Scotland.