Shell Mario

Shell Mario (also known as Koopa Mario ) is a form of Mario and Luigi that they can become by wearing a Blue Shell.

New Super Mario Bros.
This form lets Mario swim faster, and gives him the Hammer Suit-like ability to dodge the attacks of enemies when he crouches. As long as he keeps ducking, he will be safely protected from most enemies. However, it will not protect them from Bob-omb explosions, spike balls, hammers thrown by Hammer Bros. or Sledge Bros., boomerangs from the Boomerang Bros., and the Raining Debris from World 8-8.

If Mario runs for a short time, or crouches while on a downward slope, he will slide in the shell through the level. This maneuver is called a Shell Dash. Shell Mario is extremely powerful in this form, as he is able to defeat enemies such as Thwomps and Boos while doing the Shell Dash. However, the Shell Dash mechanics also make it almost impossible to Triple Jump, while Double Jumping becomes harder to perform because the player must make the second jump while running but before Mario goes into the shell to slide.

Super Mario 3D World
While the form was originally going to appear in Super Mario 3D World as a powerup, but it is replaced by weapon.

Mario & Luigi: Bowser's Inside Story
Shell Mario returns in Mario & Luigi: Bowser's Inside Story. The Blue Shell is an item found in the overworld, and it was introduced by a group of Blue Paratroopas. Mario can turn into Shell Mario by striking Blue Shell Blocks (which were spread around when Mario and Luigi hit a giant version of them). Luigi can then hit Mario using his hammer, and this will make Mario go into the shell, spinning straight forward. This can be used to hit blocks in areas which Mario and Luigi can't directly go into. However, because the shell is so heavy, Mario is unable to jump in it. If he does, he loses the shell. It serves a similar purpose to Kooper and Koops' field abilities in the Paper Mario series.

Trivia

 * Shell Mario is the only power-up from New Super Mario Bros. that isn't seen in New Super Mario Bros. 2.
 * Penguin Mario has similar abilities.
 * Despite Shell Mario making its debut in New Super Mario Bros., the Blue Shells needed to use this form were more common in Super Mario World. However, they were used like normal shells in that game.
 * In Super Mario 3D World, if a player crouches while carrying a shell, they will be able to duck into the shell and dash similar to Shell Mario.