Sidestepper

Sidesteppers are crabs that scuttle sideways, sometimes baring their teeth. Making their debut in Mario Bros., Sidesteppers were absent from many Mario games in the intervening years, but have recently begun to appear in place of standard Crab obstacles in the Mario Kart series.

Mario Bros.
The Sidestepper is the second enemy that the Mario Bros. encounter in the game Mario Bros.. Sidesteppers emerge from the pipes and walk towards the bottom of the screen. When hit from below, the Sidestepper becomes angry and speeds up. Mario or Luigi must hit the Sidestepper from below a second time to flip it over, then run into it to defeat it. If left alone, the Sidestepper will change its color to blue and flip over, making it move faster. If left alone a second time, the Sidestepper will be pink. If the last target in a phase is a Sidestepper, it will automatically turn pink. Sidesteppers also appear in all remakes of Mario Bros..

Super Mario Bros. Special
Sidesteppers also appear in Super Mario Bros. Special. Here they are credited as Chokichoki, which means "snip-snip".

Mario Clash
In Mario Clash, Sidesteppers come out of pipes and scuttle sideways. To defeat them, the player should hit them in the front or back with a Shell to turn it sideways, then hit them on the side with another Shell.

Super Mario Advance
In Super Mario Advance, when approaching Clawgrip, a short cutscene shows he is a Sidestepper, transformed into Clawgrip by one of Wart's magic bubbles.

Mario & Luigi: Superstar Saga
Sidesteppers make a cameo appearance during the battle with Hermie III. Sidesteppers run past in the background. They also appear in the Mario Bros. game included. This time, if they are flipped while red and are left alone, they become green. Doing it again makes them blue. If one is the last enemy of a phase, it will turn blue.

Mario Kart series
In the Mario Kart series, Sidesteppers can be found on beach stages like Shy Guy Beach and Palm Shore where they walk left and right. They sometimes appear with yellow highlights on their claws. Driving into Sidestepper makes the player's kart spin around. If Sidestepper is hit with a shell, star, or bomb, it flips over and disappears.

In Mario Kart Wii, during August 2009's first Competition, the player actually had to take out a certain number of Sidesteppers with items.

In Mario Kart DS, there's a mission where they're just called Crabs. It could be that they are renamed, but it could also be just an error.

Mario Power Tennis
The Mario Classic Court in Mario Power Tennis is based on Mario Bros., and the original game's sprites are used, including Sidestepper. During a Gimmick game, Sidestepper might walk across the court and trip a character.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Sidesteppers appear on the Mario Bros. stage. Sidesteppers walk out of the pipes at the top of the screen. Players can hit them from below to flip them over, and pick them up to throw at opponents. If a player is hit by a thrown Sidestepper, they fly sideways off the screen.

Super Mario 3D World
Sidesteppers appear in Luigi Bros.; an additional game featured in Super Mario 3D World. Like the other enemies in Luigi Bros., Sidesteppers behave in the same way as Mario Bros., which this game is based on.

Trivia

 * Crabbers from Super Mario Galaxy and Super Mario Galaxy 2 coincidentally appear in the same colors as Sidesteppers, although they are more red-orange and cyan instead of pure red and blue.