A Dolphinイルカ is an aquatic, goggle-wearing sea animal that made its first appearance in the Mario series in Super Mario World. They could be walked over like platforms to escape treacherous Porcupuffers.
Apart from the Mario Party series, Dolphins don't make that many appearances. However, they've been blessed enough to make cameos in the Mario Kart series and even have their own course named after them, Dolphin Shoals.
Because of their under-the-sea life, Dolphins have a very sleek and slippery body, with fins on its tail, back, and sides to help him/her swim. They have white-underbellies, while the rest of their bodies are blue. They also wear a pair of yellow and black goggles for some reason... you'd think they'd be able to see underwater already. So their probably just for fashion points.
In Mario Kart
Dolphins make their first official appearance in the Mario Kart Arcade spinoff series, as they can be swimming and jumping out of the water in Splash Circuit and Tropical Coast. They also have another vehicle named and designed after them, the Turbo Dolphin.
Mario Kart 8 and its revamp give Dolphins a much bigger role, as they actually have a course themed around them, Dolphin Shoals (2nd track in the Star Cup). The course features swimming Dolphins that perform stunts as well as stone structures shaped like Dolphins. In addition, Sunshine Airport advertises Dolphin Shoals on some of the location signs, and Toad Harbor features artwork of a Dolphin from the original title they were in, Super Mario World.
- Originally, in Super Mario World, Dolphins were listed in the instruction booklet as enemies, but were described as "not really an enemy." They could possibly take Mario in the wrong direction, but apart from that they were pretty nice. Since then, they have become more-or-less good characters.