Technically, gnolls first started as a gnome/troll hybrid, but that was temporary and quickly abadoned.Weren't gnolls originally not fiends? Were they mortals turned into fiends then or mortals brainwashed to work for fiends or actual fiends? Are there still some non-evil ones?
IIRC, gnolls originally were said to have been created by an evil god of hunting named Gorellik, with Yeenoghu (the demon lord now said to be the creator of gnolls) originally having more of a connection to evil giants. At some point it was stated that Yeenoghu killed Gorellik and took over patronage of the gnolls. 4E initially repeated this (again, IIRC, I think Yeenoghu's flail was at one point said to be crafted from Gorellik's spine), but later for whatever reason said that Yeenoghu created the gnolls, but there were some good ones that rejected his influencd (PC rules for playing a gnoll were eventually published, in fact, giving roleplaying advice such as gnolls finding the concept of politeness silly and coming across as blunt and rude even if they aren't trying to be mean).
For whatever reason, 5E decided both to dispense with Gorellik from the beginning and state that gnolls not only were created by Yeenoghu, but were essentially extensions of his demonic hunger clothed in the flesh of ritualistically desecrated hyenas.