Blue Koopa Troopa

Blue Koopa Troopas are a type of Koopa Troopa that first appeared in Super Mario World. They walk slowly back and forth on platforms. If Mario knocks them out of their shell, they will kick empty Koopa shells at Mario, instead of going inside them. If Yoshi eats a Blue Koopa Troopa, he will grow wings and be able to fly for a short period of time (or until he spits out the Koopa shell).

In Paper Mario, Mario's partner Kooper is a blue shelled Koopa. Also in New Super Mario Bros. if Mario or Luigi wears a blue shell they can retract into the shell and slide around in it and their swimming ability is increased.