A spinning Red Bloon with a black swirl. But once you see that, even if you don't, if you aim at it, or if a blast enters a certain radius, the shot will be reflected off. It creates mirror bloons, but this has 1 HP. The mirror bloons are somehow invincible, but if they leak, you lose 0 lives.
You use those Mirror Bloons to guide your shot back at the Red...Bloon...? and it will pop it.
Be CAREFUL!!!! You ONLY HAVE THREE SHOTS, AND IF IT LEAKS, GAME OVER.