This idea has been stuck in my head since I first played Pikmin.
In one level, you'll run across a key named, The Haunted Key. Have your Pikmin retrieve it. They will bring it to a rusty gate with a fence around it and will unlock it to reveal a pathway covered by eerie mist leading into a forest that gets darker the further it gets. You now have access to a side level called The Wailing Woods, a forest with dead trees, spooky sounds, perpetual night, mist covered ground and paranormal/undead enemies. The first path you should take when arriving is the one that has a sign reading, Cemetery, as your ship AI will detect strong paranormal readings. Here you will find a small, fenced-in cemetery with a spooky-looking onion in it. It will drop a ghostly seed to begin with. When you uproot it, you will have acquired a deceased red Pikmin you've lost. The thing about Ghost Pikmin is that they're immune to damage and death, but only have the abilities of red, yellow and blue Pikmin, though the rest of the colors also show up in the onion. They are useful during boss battles.