Crab

Crabs are a species of animals found in several Mario games. They are often portrayed in either the same way as their real-life counterparts or as enemies.

Donkey Kong Country (television series)
In the Donkey Kong Country television series episode Get a Life, Don't Save One, Dixie Kong owns a pet crab named Crabby. It is never actually seen on-screen (likely owing to the show's poor modeling budget), but it causes large amounts of havoc in the episode regardless.

Mario Kart 64
In Mario Kart 64, crabs can be found in Koopa Troopa Beach where they walk left and right. Driving over one will make the player spin around. Sidesteppers replace crabs in later installments.

Donkey Kong Country
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.

Mario & Luigi: Superstar Saga
In Mario & Luigi: Superstar Saga, NPC crabs appear in Oho Oasis, which can be jumped on to bury them halfways into the sand, but they get back up once you exit any of the rooms they're located in.

Mario & Luigi: Dream Team
In Mario & Luigi: Dream Team, hermit crabs appear as the main focus of the Mole Hunt game, where they must be dug up using the Mole Mario technique.

Super Mario 3D World
Tiny crabs appear in Super Mario 3D World as minor interactive elements. Two of them first appear in a sandy portion in Super Bell Hill. They are stationary in the beginning, but will start running in a certain direction when approached until they jump into the sand.

Super Mario Odyssey
Small crabs appear as wildlife in certain kingdoms in Super Mario Odyssey, specifically the Lake Kingdom, the Seaside Kingdom, and the Mushroom Kingdom. In the Lake and Mushroom Kingdoms, they are magenta, and in the Seaside Kingdom, they are yellow. Similarly to most ground-based wildlife in the game, following them for an extended period of time causes them to burrow into the sand, often leaving behind a coin.

Trivia

 * In Super Mario Sunshine one of the islands neighboring Isle Delfino is shaped like a crab. It is seen in the opening cinematic. A normal crab is visible on one of the game's group artwork.