At last, we have the last illithid-related monster in all the D&D editions (unless I actually missed something) - the Thoon elder brain. It's actually quite different from its normal counterpart - it cannot extract brains, must subsist on quintessence, has some minor magical abilities, has fewer psionic powers, has eight tendrils instead of four. I need a break.
The Thoon hulk is a large construct of mind flayer flesh and metal. Has defensive and offensive modes and can extract brains like a normal mind flayer. Has a short third arm on the chest and two normal arms that end in sharp hand-axes.
The madcrafter of Thoon is a very warped mind flayer that produces constructs within vats of caustic liquid attached to its body. One of the most disturbing images in all the Monster Manuals.
Stormcloud of Thoon is yet another construct, but this one is a flying octopoid (with six tentacles though), like those things from the Matrix.
Scythers of Thoon are more advanced construct troops that wield dual-wield scythes, have headlights eyes, can shoot fire beams out of the eyes, and dispel magic with a touch. Cool.
This is the most common soldier employed by the mind flayers of Thoon. It's a construct that has five different combat modes, each announced loudly and clearly upon activation.
The Thoon infiltrator is a human enhanced by a set of metallic, parasitic tentacles on the back of his neck. They keep him alive, let him fire poisonous darts, infect others, and cast spells.
Thoon thralls are humanoids captured by the mind flayers of Thoon (usually via not-yet-statted Thoon infiltrators) and turned into sleeper agents. The sample statblock assumes a very normal human.
The shadow flayer is a product (an imperfect one) of the Thoon-aligned mind flayers' breeding program. They are more agile, less psionically powerful, but can briefly turn invisible.
All right, let's forget about taking a break and just finish this. The first of the mind flayers of Thoon is a normal mind flayers with clerical powers granted by Thoon. Wears plate armor and flails his flail around. This is simple, I didn't even have to create a new racial template. The other ones will be more difficult.
And now the bigger variant of the seugathi that replaces its magical abilities with psychic powers. That's the last of the Pathfinder exclusive illithid-related monster. There's only 10 of them left (mind flayers of Thoon), but I probably will take a break from them.
A tangentially related to the illithids monster. The seugathi are created when a neothelid performs a self-impregnation ritual. They are wormlike beings with magical powers and two prehensile tails that can be used as arms. Pathfinder exclusive.
Now let's convert a bunch of illithid symbionts from Dragon Magazine #308.
Here we have the oblex, an illithid-bred ooze that can create simulacra of creatures whose memories it eats. It's a newcomer that was introduced in Mordenkainen's Tome of Foes for D&D 5e. The spawn in the book reuses an old art for a different creature (graveyard sludge or something).
Another obscure illithid-related monster that is called octopin despite having only six tentacles. Honestly, I like this one a lot, it differs enough from "normal creature, but with several illithid abilities spliced onto it."
The kezreth is a combat transport created from a decapitated illithid. Can transport 15 human-sized creatures with ease and blast things with a telekinetic beam.
The neothelid overlord is a Pathfinder-exclusive monster - a neothelid that splits its consciousness both mentally and physically and gains supreme psychic powers in addition to its psionic arsenal and an aura of madness. Pretty cool.
The nyraala golem is an amalgamation of psionically-bound flesh and fungus that can disassemble itself into a thin fungal film for camouflage. Employed by the illithids as an alternative to the brain golems.