Dolphin

Dolphins are goggle-wearing mammals that jump out of the water in Super Mario World.

Super Mario World
Dolphins only appear in Vanilla Secret 3 and in a hidden area near the end of Chocolate Island 1. They are used as platforms to cross the water infested with Porcu-Puffers.

Mario Party 4
A Dolphin appears in Koopa's Seaside Soiree in Mario Party 4, where he will let the character played as get on his back while he swims through the water. The character can Jump to get Coins this Dolphin swims by on the trip.

Mario Party Advance
An unnamed, comedian Dolphin is met in Mario Party Advance; one of the player's quests involves helping this Dolphin get his confidence back by listening to his jokes and laughing at them. Occasionally, several Dolphins will appear and form an audience to listen to this comedian Dolphin's jokes.

Mario Party 7
In Mario Party 7, some Dolphins can be seen jumping out of the water in the file select screen.

Mario Party 8
In Mario Party 8, the Dolphins play an important role in the board Goomba's Booty Boardwalk‎; players can pay them a certain number of coins depending on what place they are in, allowing them to take a ride on the Dolphin's back and advance a number of spaces.

Super Mario World television series
In the Super Mario World television series, Dolphins appear in the episode "Mama Luigi". In this episode, two Dolphins, seeing that Luigi and Yoshi are being attacked by a Torpedo Ted, decide to help the two by letting them ride on their backs. Evading the Torpedo Ted, the Dolphins drop Luigi and Yoshi off at a nearby beach. After being thanked by Luigi, the two friendly Dolphins leap back into the water, cheering happily.

Trivia

 * As of Super Mario 64 DS and New Super Mario Bros., Dorries were always seen wearing goggles, making their faces greatly resemble to those of Dolphins.
 * In the Japanese version of Super Mario World and Super Mario Advance 2: Super Mario World, Yoshi can eat Dolphins, although he cannot do this in the American SNES version of Super Mario World.
 * A working name for the Nintendo GameCube was "Project Dolphin"; the strange and unused name resulted in many Dolphin references in various games. Mario game examples of this include the fact that Isle Delfino (the setting of Super Mario Sunshine) is shaped like a Dolphin and Dolphin-themed, and the possible name "Team Dolphin" given in Mario Party 7. However, these references are technically given to the dolphins on Earth rather than the Mario ones.
 * A medium-sized bike in Mario Kart Wii, the Dolphin Dasher, is based on a Dolphin.