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-like legs suspend the Onions out of the reach of hungry predators, and provide access to the inside for Pikmin. In Pikmin 1, Olimar states that he inspected the Onion's legs, and saw they were covered with tiny little hairs. 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 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.
The Red Onion in Pikmin 1 while dormant.
The Red Onion getting ready to Stand up.
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The Red Onion in midair.
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All three Onions on the ground.
When the Onions were first discovered in Pikmin 1, they were dormant. They had a dark grayish color, were flowerless, and their legs 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 dormant until conditions improve.
At the end of Pikmin 1, there is a rather confusing part where fourteen Onions, of colors that have not appeared in a Pikmin game so far, fly above the Planet of the Pikmin. It is unknown whether they are trying to follow Captain Olimar or merely hovering for the night. This could mean that more colors may yet be introduced in Pikmin 3. Interestingly, one of the Onions seen is purple, and Purple Pikmin were indeed introduced in Pikmin 2, but without an Onion.
In Super Smash Bros. Brawl
In Brawl, the 3 Onions appear as a trophy and on the "Distant Planet" stage. Here, when a pellet is thrown into one, an item is ejected. More items appear if the pellet and Onion have the same color, similar to the way it works in the Pikmin games.