Dark Pikmin are the final Pikmin found in Pikmin SP. They do have an onion, but they are only needed for one thing: the final boss. They are the only Pikmin that can attack him. The boss can create his own Dark Pikmin, however, and they will attack both you and your Pikmin, even if you have some Dark Pikmin. There are 100 sprouts outside their onion when you find them. They are located within your landing site, and the worst part is that you cannot pluck them without the Pluckaphone upgrade that you receive from the Diamond Tweeter. These Pikmin can be sprouted via Shadow Candypop Buds, but the buds are heavily guarded by enemies from all sides. These Pikmin are made out of dark matter, they have dark black flowers, with blue centers, and they have an eerie red glow, with deep purple eyes. The pikmin that the final boss makes, however, look a little different. These Pikmin are found in the final level, Path to Darkness. You will only get a glimpse of their candypop buds in the only cave of this level, which is The Shadow Nest.
The boss's version of Dark Pikmin: