Mosquitus

Mosquitus (モスキータス Mosukītasu?) is a giant bug mini-boss in Mega Man X3, appearing in the second stage of Dr. Doppler's lab; but only if Vile was destroyed earlier. Mega Man X can encounter it, but if the player battles it with Zero, X can gain the Z-Saber. If the Mosquitus is defeated by Zero, the monster will catch him in a death grip and explode, leaving him severely injured. This causes Zero to be disabled for the rest of the game and pass his weapon onto X, but this will also result in the less-than-ideal-of-the-two possible endings.
Mosquitus, while holding X, can be defeated in one hit with a charged Tornado Fang.