Monster: Intellect Devourer

Monster: Intellect Devourer

The intellect devourer is a body snatching brain on legs. These are sometimes used by the mind flayers are spies.

Monster: Phthisic

Monster: Phthisic

Yet another monster that is said to be associated with the mind flayers, but its description has nothing even hinting at it. Basically, it's a spiky, psionic troll spawned from somebody's psyche.

Monster: Ineffable Horror

Monster: Ineffable Horror

The Monsters of the Mind article in Dragon Magazine #337 has a sidebar that lists monsters related to the mind flayers. The ineffable horror is one of them. I've read through its description, and the only mind flayer-related thing is that they somewhat have similar appearances. Maybe I'm just missing some pre-3.5e lore, if these monsters appeared there.

Monster: Mindwitness (Half-Illithid Beholder)

Monster: Mindwitness (Half-Illithid Beholder)

The worst/best of the worlds of magic and psionics - the combined mental might and a nasty outlook on the world of an illithid and a beholder.

Monster: Mind Worm

Monster: Mind Worm

The mind worm is a terrifying monster that can view you remotely, then snag you through a mirror or other reflective surface, then drain your HT, then swallow you from afar, then digest you.

Monster: Ustilagor

Monster: Ustilagor

Ustilagors are larval intellect devourers. They are almost mindless, but still possess some psionic abilities. They have a symbiotic relationship with a particular species of fungus that keeps it moist and protects against fungal afflictions. Ustilagors eat various fungi by dissolving and absorbing them through the skin, and are eaten by the mind flayers as a delicacy. Also, the ustilagors really like gems and attack everyone who carry them.

Monster: Nerve Swimmers

Monster: Nerve Swimmers

Yet another aberration that was created by experimenting on immature illithid tadpoles. They can parasitically possess people and are quite cruel in general.