Millimpede

The Millimpede is a species of pillimpede that is practically invincible because of its exoskeleton and constantly roams its territory.

=In fanon games=

In Pikmin: Infinite Exploration
The Millimpede is a massive millipede-like creature in Pikmin: Infinite Exploration that crawls around in dark areas with many corridors. The Millimpede is much less an actual enemy as it is a moving hazard, as the Millimpede can not be killed, and attacking it has little to no effect. The Millimpede is attracted to sources of light, and so it targets leaders, glowcaps, beams of light, and lit lightbulbs. Land-based creatures that live in the tunnels that Millimpedes call home will flee or attempt to hide if they notice that the Millimpede is approaching. Any creatures that do not move out of the Millimpede's route will be trampled by it as it passes through.

There is no way to defeat the Millimpede, but it can be deterred if Pikmin are thrown into its eyes enough times, which is very dangerous and only works temporarily. It would be wiser to look for alcoves to hide in should the Millimpede find their group, as the Millimpede can not only wipe out a full group of one hundred Pikmin just by running over all of them but also take leaders out quickly as well.