While not in the Fiend Folio, the Githyanki entry mentions them. Of course, it also mentions red dragons, but I don't want Smaug hanging around.
I've never cared for all the complicated backstory built up around the Illithid, especially in the second edition monster manual, so I'm rewriting it.
Gelatinous. If squeezed hard enough they pop, squirting Jale ichor everywhere. The ichor smells sweet. If consumed, save vs. poison (or whatever is appropriate for your system. For BoL, make a Might check). If the save fails, you enjoy 1d4 hours of agonizing abdominal cramps, vomiting, and diarrhea. Imagine the worst food poisoning you've ever had multiplied by a billion. If the save succeeds, you experience 1d10 minutes of mild nausea followed by 1d4 hours of intense hallucinations. The hallucinations are similar to the effects of DMT but they go on for far, far, far too long. After your mind clears, you've learned enough Aklo to get by as a tourist.
Flabby. Imagine a cross between Baron Harkonnen, a cuttlefish, and one of the viler Borgias.
Translucent. Their skin is a pale purple, with mauve liver spots, at about 20-30% opacity. The mauve spots are at about 40-50% opacity, but deeply saturated. The jale ichor is visible, pulsating and pumping about.
Decadent. Like Hedonismbot, but not funny.
Territorial. Mindflayers have long lives, spawn infrequently, and prey on sentients. As a result, they tend to be intensely territorial and competitive.
Paranoid. Like James Jesus Angelton on a month long amphetamine binge.
Psychopathic. At least by analogy. They are incapable of experiencing empathy, remorse, or guilt.
Mind-eaters. Not brains, but minds. Same mechanics as eating brains, at least as far as how many rounds and whatever. Instead of a cracked open skull and a corpse, the result is a drooling husk capable of obeying simple commands. Mindflayers don't need to eat minds, but it gets them high and fuels their sorcery.
Related to Mi-Go, or at least from the same region of space-time. Their molecules vibrate at a different frequency or they're made of astral matter or whatever crazy explanation you prefer.
Reproduce with a cloud of spores. Mindflayers lay grotesquely swollen eggs that pop emitting a noisome gas that's really just a mass of egg-spores. The spores get in your lungs, causing coughing and internal bleeding, until you start to swell up and enter a coma and turn all squidfaced. The bleeding is from all the egg-spores fighting it out in your lung until just one wins and takes over your body.