Suckerpunch
The Suckerpunch is seemingly a marine creature with no visible facial features. It looks like a cylindrical mass of spongy material that wobbles back and forth, shuffling slowly towards its prey. Once it is in range of eating Pikmin, it leans over and suctions nearby Pikmin straight into its mouth. Because the "mouth" is on top of the creature, throwing Pikmin on top of it is a bad idea. At the same time, you run the risk of it eating Pikmin if you attempt to swarm it. The trick is to get it to bend over to try and eat Pikmin, then quickly maneuver them out of the way and throw them on the bent part of the creature before it reverts back to its normal cylindrical form. This way, the Pikmin can still attack it at that point once it goes back to normal cylindrical form. They'll continue to damage it until it is defeated, as the Suckerpunch can't shake off Pikmin that are latched on to it, only eat Pikmin situated around its base. Upon defeat, the Suckerpunch's body collapses on itself, then turns into goo and dwindles away. Suckerpunches are slightly smaller than fully-matured grub-dogs.
Trivia
The Suckerpunch bears a striking resemblance to the infamous "Like Like" enemy from the Legend of Zelda series.