Dolphin

Dolphins are goggle-wearing cetaceans that debuted in Super Mario World. Here, they act as platforms that leap out of the water, helping the playable character reach items or avoid enemies. In other Mario-related media, Dolphins typically appear as background elements, though they have more prominent roles throughout the Mario Party series and a particular Dolphin even has a playable role in Dr. 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 a body of water populated by Porcu-Puffers. They jump in and out of water either vertically or in parabolic arcs. In the Japanese version of Super Mario World, Yoshi can eat the Dolphins. This was changed in Western versions of the original Super Mario World, but was retained in all versions of Super Mario World: Super Mario Advance 2. They also appear in the opening sequence that was added to Super Mario World: Super Mario Advance 2.

Super Mario World television series
In the Super Mario World television series, Dolphins appear only in the episode "Mama Luigi". Seeing that Luigi and Yoshi are being attacked by a Torpedo Ted, two Dolphins decide to help them 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.

Nintendo Adventure Books
A Dolphin is featured on the cover of Dinosaur Dilemma, and artwork of one is used in the book, but none appear in the actual story. An equally friendly sub-species of them, the Micro-Dolphins, is discovered to be living inside Yoshi in Unjust Desserts.

Super Mario-kun
Dolphins appears in the second chapter of the second volume of Super Mario-kun as allies. When the giant Bowser robot attacks Mario, Luigi and Yoshi at Vanilla Dome, he creates a hole in the roof, flooding the dome with the water of Vanilla Secret 3. Dolphins come down with the water and offer to help the three heroes to escape by making them riding their backs and swim over the waterfall, reaching the surface. Mario then proceeds to clog the hole, trapping (briefly) the Bowser robot in the dome and then proceeds to thank the Dolphins, along with saving them from Yoshi (who kept trying to eat them).

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
A unique Dolphin is met in Mario Party Advance; several other Dolphins also appear and form an audience to listen to his jokes.

Mario Party 5
In Mario Party 5, a Dolphin has been seen in the Undersea Dream. It only shows up if the player lands on the Green Space on the sunken ship.

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.

Mario Party 9
In Mario Party 9, Dolphins appear on Blooper Beach. On the first half of the board, a Dolphin is the first active sea creature when a party is started, being six spaces away at first. They advance two spaces every turn, and if the player lands on or catches up to the dolphin while captain, the Dolphin gives them 5 Mini Stars. Dolphins also appear when the player lands on a Lucky Space near the end of the board. Landing on one of them results in a trio of Dolphins leading the player to an island with several Mini Stars. Additionally, during the minigame Ring Leader, the players ride Dolphins to jump through hoops.

Mario Party: The Top 100
In Mario Party: The Top 100, a Dolphin makes a cameo appearance in the Collection mode, appearing in Mario Party 9's box art.

Mario Kart Wii
While no real Dolphins themselves appear in Mario Kart Wii, a medium-sized bike called the Dolphin Dasher is based off of a Dolphin.

Mario Kart Arcade GP DX
Dolphins make their first cameo appearance in the Mario Kart series in Mario Kart Arcade GP DX, where they can be seen jumping out of the water in the Splash Circuit and Tropical Coast stages. A kart based off Dolphins called the Turbo Dolphin appears in the game.

Mario Kart 8 / Mario Kart 8 Deluxe
In Mario Kart 8 and Mario Kart 8 Deluxe, there is a banner with Dolphin artwork from Super Mario World on it found in Toad Harbor. Also, Dolphin Shoals features Dolphins that act similarly to those in Mario Kart Arcade GP DX, as well as Dolphin-shaped hill structures. In Sunshine Airport, this course can be seen listed on the signboards in the terminal next to other courses.

Mario Golf: World Tour
Dolphins appear in Mario Golf: World Tour swimming around in the background of Cheep Cheep Lagoon.

Mario Playing Cards
A Dolphin appears on the three of clubs in the NAP-06 deck of the Super Mario Bros. 30th Anniversary release of the Mario Playing Cards.

Dr. Mario World
Dolphins appear in Dr. Mario World as assistants. Their stage mode skill increases the player's base score by 50 points for every blue virus they eliminate. Their versus mode skill grants a 20% chance an exploder will appear when all viruses in the player's stage are eliminated. These numbers can be increased by obtaining duplicate Dolphins in Staffing.

A Dolphin also appears as a playable character under the name Dr. Dolphin, marking the first time a Dolphin has been playable in any Mario game.

Other appearances
In the Super Mario Mash-Up Pack of Minecraft, regular dolphins are reskinned as Dolphins from the Mario franchise.

Assistant

 * Stage mode: "Increases base score by 50/70/100/150/200 points for every blue virus eliminated."
 * Versus mode: "If all viruses in your stage are eliminated, grants 20%/40%/60%/80%/100% chance an exploder will appear."


 * Dr. Mario World Twitter: Yellow: "The second doctor coming May 28, 2 AM PT is Dr. Dolphin! This kind doctor will let you ride on their back when you're in a pinch. Just be careful not to let your bag get wet...and please don't hurt me."

Trivia

 * The fish enemies Blurp and Bub are visually comparable to the dolphins.
 * Isle Delfino, the setting of Super Mario Sunshine, is shaped like a dolphin.
 * In Super Mario 64 DS, Dorrie was redesigned with elements seemingly influenced by the dolphins. This design is the basis for all of Dorrie's subsequent appearances.
 * The rival character Guppy from Super Mario Galaxy was initially envisioned as one of the dolphins from Super Mario World. However, as his role was finalized as an intimidating bully with a wild nature, they shifted his design to that of the new orca character seen in the final game.