Dorrie

Dorries are large, friendly sea dinosaurs, resembling Plesiosaurs and Elasmasaurs, that were first seen in Super Mario 64.

Super Mario 64
The first known Dorrie from Super Mario 64 lives in an underground lake in the Hazy Maze Cave. He spends his days swimming in circles, but will gladly give Mario a ride if he gets on his back. It is also possible to steer Dorrie by facing the desired direction on his body. Ground Pounding on his head will cause him to lower his neck. Strangely, a sign near the lake in which he takes residence states that he might eat visitors if they get too close, although he otherwise proves to be a harmless creature.

Super Mario 64 DS
Dorrie also, of course, appears in Super Mario 64 DS. However, in the game, he wears goggles, and more resembles the Dolphins of Super Mario World. He fulfills the same role as in Super Mario 64.

New Super Mario Bros.
Dorrie returns in New Super Mario Bros., and has goggles like in Super Mario 64 DS. Mario can ride on his back to cross large bodies of toxic water in certain levels of World 4. If Mario ground pounds Dorrie, he will move faster through the water.

Mario Party series
Dorrie is a reoccurring minor character in the Mario Party series, although only directly appearing in a few games, such as Mario Party 3 and Mario Party Advance; a lot of aquatic equipment in the Mario Party series, such as boats, are based on Dorrie. In Mario Party Advance, a Dorry reveals that he is lonely after scaring away Mr. E. If Yoshi comes, they'll become friends and Dorrie gives him a Gaddget. The credits show that Yoshi ought to visit him soon. It says that he hasn't seen him in a while, or as it says, ages.

Trivia

 * In the Japanese version of Super Mario 64, Dorrie is referred to as "Doshi", most likely a parody of the Loch Ness Monster (who is commonly referred to by the nickname "Nessie", which is pronounced "Neshi" in Japanese) and Yoshi. In the game's German version, he is actually called Nessie. Also, the English version of where he is in Mario Party Advance is punned on Loch Ness, Scotland, where Nessie comes from - Loch Dorrie.
 * Although Dorrie is male, Mario Party 3 uses feminine pronouns in some minigames featuring him.