The
Gnoll has a unique history. From 1974's
Monsters and Treasure Volume 2 by Gary Gygax and Dave Arenson (this is all on a wiki):
GNOLLS: A cross between Gnomes and Trolls (. . . perhaps, Lord Sunsany (sic, should be Dunsany) did not really make it all that clear) with +2 morale. Otherwise they are similar to Hobgoblins, although the Gnoll king and his bodyguard of from 1 - 4 will fight as Trolls but lack regenerative power. Lord Dunsany and another writer St. Clair used the term "gnole" and described them as woodland creatures that spy on you from trees. A senior gnole has tentacles rather than arms and no ears, with small red gemstone eyes. Elves spoke gnoll, hinting at woodland nature.
1977, Gnolls get a makeover. AD&D invests in them hardcore as a common humanoid race. Yeenoghu becomes the Demon Prince of Gnolls and provides our glimpse of hyena folk. They are now "rapacious," lazy, subterranean monsters on great terms with most other evil humanoids. No trolls, no gnomish size, no woodland. They're also large.
1989/1993, Gnolls get more evil. They will eat anything meat, including other races, and they like hearing you scream as they eat you. They are ecological nemeses, eating everything in sight and moving on. In
3rd Edition, they keep nasty and hate giants too. But, they shrink from Large to Medium. In
2005, we get the option to play gnolls, because Drizz't did it so why not? Is that why they had to shrink?
2008, 4th Edition, gnolls remain medium murderhobos. In 2008, we get Dungeon Magazine #367 that explains Yeenoghu created gnolls by feeding demons to hyenas. However, although gnolls are basically demon lunchmeat packaged in a hyena body, some of them figured out how to be good. Oh, and we got some cool art for these shrinking, most-of-the-time bad guys that once were gnome-trolls.
5E came back full circle and gave gnolls some loving in Volo. They're back to being their vile, vicious selves who repopulate in an insane way.