waterwraith
Scientific name: Amphibio sapiens
A truly bizarre and terrifying creature from Pikmin 2. Resembling a humanoid steamroller made from water, it is effortlessly able to roll over all of your Blue Pikmin, since blues are the only pikmin you can bring to its cave, the Submerged Castle. And you can’t even harm it either, since as an apparition, it is anchored to another dimension.
The above point about it being invincible is true until you reach the bottom of the Submerged Castle. There, candypop buds can convert your Blues (and Bulbmin) into Purple Pikmin, whose gravitational waves are capable of making the Waterwraith vulnerable. In fact, once you break its rollers, it becomes mostly harmless, and can be easily defeated then.
The Waterwraith makes its return in Pikmin 4, this time terrorizing a very similar cave called the Engulfed Castle.