Spiny Shell

The Spiked Shell, otherwise known as the Blue Shell, is an item that appeared in Mario Kart, but has the effects of it in Double Dash. The Spiked Shell has spikes and wings. It will follow the player in first place or second place (if you are in first). When it hits a character, it will cause an explosion that damages all but the player that threw it.

In Super Smash Flash 2
If an opponent grabs it and throws at someone, the shell will fly away and hit the player again with good damage and knockback.