Susurrus
Image from Fiend Folio, © TSR Inc.
[The word "susurrus" means a whispering or rustling sound, and is sufficiently obscure that spellcheck doesn't recognize it. So I'm willing to use that as the name of the monster (even if it was misspelled in its initial appearance), but if that sort of thing bugs you I have some alternatives. The susurrus appeared in the 1st through 3rd editions of D&D, and although the mechanics were similar in all three versions, every time what it was was reinvented. The 1e susurrus is said to have an exoskeleton, and between that and its filter feeding in air seems to be some sort of sponge-like creature. The 2e version is made of bamboo. The 3e version is made of glass. I went back to the original, but explicitly made it a glass sponge. And the warnings I got doing lab work with glass sponges back in college inspired me to make it shed sharp little glass shards in addition to just cutting things that touch it.
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Susurrus CR 7 N Aberration This ape-like, headless creature appears to be made out of a fine lattice of translucent glass. As it moves, air whistles through the holes in its body, creating a faint, eerie whispering sound.
A susurrus by any name—hushcreeper, whisper beast—would be a sapient glass sponge that feeds on organic matter filtered from the air. They are generally peaceful, perching on mountain crags or canyon walls when the wind is up in order to gather pollen, fungal spores, tiny insects and bacteria. The sound made by the air whistling through their many tubes and hollows is eerie and soothing all at once, and causes drowsiness in creatures that come too close. This is effectively a warning to anything that would attempt to attack it; a susurrus’s body is covered with tiny spicules of razor sharp glass, and its fists hit with the force of a battleaxe. Creatures struck have glass fragments embedded in their bodies, causing excruciating pain until the glass is removed.
Susurruses despise smoke, as it interferes with their filter feeding and tastes terrible besides. Parties carrying torches or lighting bonfires in a susurrus’s territory may find themselves assaulted and driven off. They are also uniformly hostile towards corporeal undead, attacking them in a rage whenever their paths cross. The dronesong of a susurrus can immobilize the undead, making them a powerful ally to undead hunters if an accord can be reached. Necromancers tend to despise susurruses, as a single one crossing the path of their armies can debilitate them effortlessly, and may hire adventurers under false pretenses to exterminate the glass creatures.
A susurrus can live for thousands of years if not slain through violence. They rarely reproduce, and do so through fission, breaking off a chunk of their body and planting it in a windy area to grow to full size. Their own language is difficult to reproduce by organisms without multiple mouths, but they do understand and speak simple phrases in Aklo.