Conscious Eater

While this creatue may be a floating brain, it is yet unknown where these brains come from, or even if they're from humanoids at all. They aren't aggressive, but it's still advised to stay away from them if spotted, especially if you're alone. If one isn't careful, a person can end up forgetting weeks, or even years of their life in a short amount of time. These creatures feed on memories and if caught by them, will start to slowly drain a person's memories. For reasons unknown, these brains don't disintegrate like other monsters do, which further proves that they might have once been humanoid brains.
Thankfully they don't travel in packs, though there might sometimes be pairs of them here and there.
There was once an attempt to try and observe these creatures at a long distance to find out if and/or how they ever reproduce. One Pale Druid went missing and was finally found caught by one of these creatures. Once they freed him they realized he had lost about 20 years of his memories. That particular Conscious eater then didn't continue to try and feed on anyone for about a month afterwards, even if people went closer to it. It was almost completely passive. No evidence from that study was found that they procreate with each other, and the theory right now is that they somehow form from humanoids or intelligent monsters.