Ustilagor.... what can we say... not exactly a classic old-school monster... To tell you the truth, before this article, I had no idea what this even looked look, having never heard of it.
From the "semi-sentient fungus" files, it looks kinda like a brain which can scuttle around like a crab... with caustic alkaline (not acid!!!) tendrils that can whip you in the face. Definitely a creative concept.
While not huge game-changers for a campaign, they definitely provide another interesting random monster encounter for the psionic adventure.
I get the definite impression these things would exist in a symbiotic relationship with some other more-intelligent monsters. Perhaps they are created or cultivated by Mind Flayers, Aboleths, Duergars, or Githyanki as some sort of guardian pets. [Later editions turned this thing into an Intellect Devourer larva... I'm not sure I'd go in that direction.]
attacks: 1 tendril slap (2-5 damage + caustic alkali)
Psionic Ability: 155-180
attacks: Id Insinuation
defense: Mind Blank + immune to all but Psionic Blast
Telempathic projection: hate or distrust for associate, fear of fungi, loathing of area, uncertainty