Green Shell

The Green Shell is a throwing item that appears in Super Smash Flash and Super Smash Flash 2. It is one of four items from that conform the shell family, which also includes the Red Shell, the Yellow Shell, and the Spiny Shell. It keeps moving in one direction unless it hits a wall and then bounces in the opposite direction. It is an item from the  universe.

In Super Smash Flash
The Green Shell is referred to as simply as "Shell" in Super Smash Flash due to the Red Shell's absence, it appears commonly and can be thrown, however, it will keep moving and will ignore any wall of object in front of it and will simply disappear once it reaches the blast line, it cannot be stopped nor reflected.

In Super Smash Flash 2
The item got revised for Super Smash Flash 2, where it can be used by both picking up the Shell and throwing it, or by jumping on it. It is capable of damaging all players, including the one who used it. It deals 11% of damage and has a moderate knockback.

Origin
In Super Mario Bros., there are turtle-like enemies called Koopa Troopas. When the player jumped on a Koopa, it would hide in its shell for a short period of time. When the player kicks the shell, the shell slides across the ground knocking whatever is on its path (which could include the player). It would also slide across the ground if the player jumps on the shell. When the shell hits the side of a wall or pipe, it ricochets and slides in the opposite direction. Koopa shells were also an item that could be pulled out of the ground and thrown at enemies in Super Mario Bros. 2. In Super Mario Bros. 3, the player could now pick up a Koopa shell and throw it at enemies. In , if the player jumps on a Koopa, rather than hiding in its shell, it instead gets knocked out of the shell completely; leaving the shell empty. There were also instances in Super Mario World where a player can just find an empty shell. In the  series, the empty Koopa shell behaves the same as it did in the games mentioned before. A player can pick up and throw empty Koopa shells to slide across the stage to do damage to enemies, and the shells also ricochet off of walls. Also, the most common shell color for a Koopa is green.

Following its appearances in the Super Smash Bros. series, this shell behaves similarly in the  series of fan games but was a bit limited in the first Super Smash Flash and coming fully functionable in the reboot, Super Smash Flash 2.

Trivia

 * Prior to the Beta version, all Shells shared a single spot in the item switch. Their description read: Throw them at opponents and watch them slide!.