Crab

Crabs are a species of animals found in a few Mario games. They are often portrayed in either the same way as their Real World counterparts or as enemies.

Super Mario Sunshine
Small crabs populate the beaches of Super Mario Sunshine. They have no effect on the gameplay and are simply used for decoration.

Mario Kart series
In the Mario Kart series, Crabs can be found mostly on beach stages where they walk left and right. Driving over one will make the player spin around. Sidesteppers replace Crabs in later installments.

Donkey Kong series
In the Donkey Kong Country remakes for Game Boy Color and Game Boy Advance, Donkey and Diddy Kong may encounter a crab during a challenge at Funky's Fishing. The crab has no effect on the Kongs' score, but if they can succeed in hooking it, they will earn themselves a special Photograph from Funky Kong. It scurries across the bottom of the water rather quickly, and appears randomly, so the Kongs should always keep an eye out for it. Once the crab is caught, it will never appear again.

Also, in the underwater coral-themed levels of Donkey Kong Land, several crabs can be seen staying on the coral reef. However, they do not harm nor affect the primates in any way, as they simply make part of the scenery.

Trivia

 * In Super Mario Sunshine one of the islands neighboring Isle Delfino is shaped like a crab. It is seen in the opening cinematic.