From the Super Mario Wiki
A Crystal Creature is an offspring of the Gem-Bodied Creatures that Wario faces during the events of Wario World. They resemble smaller versions of Gem-Bodied Creatures, but with a distinct face. The main enemy will summon two to five of these enemies to attack Wario. Fortunately, Wario can use these to damage the Gem-Bodied Creature, which is immune to his punches.
Red Crystal Creatures wield a sword that can slice Wario. Yellow ones spin in a circle towards him. Blue Crystal Creatures stab repeatedly in front of them, hoping to skewer Wario. Flying Crystal Creatures of any color simply dive-bomb him. The ground-bound Crystal Creatures block all frontal attacks, so it's easier to stun the flying ones.
When the main enemy is defeated, all undefeated Crystal Creatures disappear.