also throwing down on the pile, gnolls are indeed spawned from gorged hyenas. It's in Volo's Guide to Monsters however, not in MM.
At one point I did a crap ton of research on Hyenas; they are incredibly intelligent, and have highly sophisticated social structures within their packs. I just now looked up "Volo's" and for heaven's sakes I could reproduce my old write up and it would be both truer to the idea of "hyena" AND a heck of a lot more interesting.
