1. These creatures are not called "monsters", but "enemies"
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2. They are not antagonists of the Pikmin franchise (apart from Titan Dweevil, as he intended to exterminate the entire Pikmin species using weapons, which counts as genocide, Plasm Wraith and Captain Olimar's love, but he kind of could be antagonistic, as he summoned many animals (maybe he wanted to summon them to increase the population for nothing) and wanted to kill all the Pikmins and captains for trying to "burn the film" on Olimar's relationship, Man-at-Legs, is a cyborg animal that wants to kill Pikmins and captains for no reason and Waterwraith, a cowardly amoeba who uses rollers to kill without reason).
3. They are not 100% aliens, but animals and earthling aliens, since the Pikmin franchise takes place on planet Earth (confirmed by Shigeru Miyamoto).