Onion
The Onions are the nests of the Pikmin. Olimar named them so because of their resemblance to a vegetable native to his home planet. These bulbous plant-hives have brightly colored bodies with a black and white band across the midsection. Three stilt-legs suspend the Onions out of the reach of hungry predators and provide access to the inside for Pikmin. There are three known types of Onion, each with unique coloration corresponding to either Red, Yellow, or Blue Pikmin, and only one of each has ever been seen. Subterranean Pikmin species such as Purple Pikmin, White Pikmin, and Bulbmin do not have Onions, or at least none that have yet been discovered.
During the daytime, Pikmin hunt and bring food, such as pellets or slain beasts to the Onion, which in turn produces new Pikmin seeds; if there are fewer than 100 Pikmin on the field, the seeds are ejected and take root in the ground, but if not, they are stored inside the Onion, along with any other Pikmin not on the field. When night falls and all the Pikmin have returned, the Onion folds its stilt-legs and lifts off from the ground, presumably using its flower petals like rotor blades to propel itself into the low atmosphere where it hovers until daybreak. If an Onion loses all its Pikmin, it will produce a single seed the following day to prevent extinction.
When the Onions are first discovered in Pikmin 1, they were dormant. They had a dark grayish color, were flowerless, and their stilts were buried in the ground until Olimar came upon them, at which point they sprang to life and spat out a single seed. It is not known exactly what triggers this hibernation, though it's possible that a loss of Pikmin or food shortage may prompt the Onion to become latent until conditions improve.
In the Pikmin games, there is one onion for the 3 dominant species of Pikmin, Red, Yellow, and Blue. There are no onions (at least, so far) for Purple or White Pikmin, or Bulbmin. Purple and Whites are stored in the Ship, while Bulbmin cannot be taken out of subterranean caves. It is not known if there is a purple and white onion, or if the two variants are just mutations that have no onion attached to their 'species'.
At the end of Pikmin 1, there is a rather confusing part where dozens of Onions of colors that have not appeared in a Pikmin game so far fly above the Planet of the Pikmin, but it is unknown whether they are trying to follow Captain Olimar or merely hovering for the night. This may mean that more colors are yet to come if Pikmin 3 is being made.