Withdraw is Squirtle's Side Special Move. Only seen in a screenshot, it appears that Squirtle goes inside its shell and speeds across the stage, powered by a jet of water shooting out the back of its shell. It appears technically similar to Yoshi's Egg Roll. However, if someone jumps on top of Squirtle, it gets stopped and Squirtle falls into a laggy animation. While Squirtle is in its shell it is invincible, that is usefull to counter some final smash. Unlike Yoshi's Egg Roll, while using Withdraw, Squirtle can't change the direction. But is as Koopa Troopa's shell, if someone hits Squirtle's shell it will change its direction and its speed.
Origins
Withdraw is a move in the Pokémon games learnable by shelled Pokémon that raises their defense by withdrawing into their shells.