Strangely enough, James Jacobs, the Dungeons and Dragons 3.5-derived Pathfinder publisher Paizo's creative director, answered much the same question about burnt othur fumes (which originally appeared in the Dungeon Master's Guide, Third Edition (2000)) in 2014 in this thread:
Burnt othur fumes are an awesome poison. What's othur though? Is it a plant, a creature? Googling has helped me naught and the books don't actually describe any of the poisons fluff-wise.
They're poison, that's what! Don't breathe that!
One of the design philosophies of the original 3rd edition rules, and one I very much did not agree with and argued against during the alpha for 3rd edition (but obviously lost said argument) was the decision that "D&D is set in a fantasy setting and as such the diseases and poisons should be fantasy diseases and fantasy poisons." Which on one layer is fine, but when you're presenting sample diseases and poisons without any flavor text whatsoever as raw rules is really pretty dull.
But that's why in 3rd edition all the diseases have made up names like "slimy doom" and "filth fever" and aren't real-world diseases like "bubonic plague" or "leprosy." The same thing went with poisons.
So... just as there was never really any description of what the actual symptoms of diseases like filth fever or slimy doom were, we never got descriptions of what othur is and why when you burn it, it gets poison.
The fact that we didn't fix this problem when we were pulling the poisons and diseases into the Core Rulebook is unfortunate, but they were among the last parts of the book that went in and by that point we were running short on both time and space. The idea was that at some point we'd clarify them in a campaign setting book... but since these things have been a part of the game for over a decade now, they've kinda slipped again and again through the cracks.
For what it's worth, I've always thought of othur as a powder distilled from numerous different poisonous plants and ground together that remains inert until it is burnt.