Some Ral Partha and Citadel gnolls

Gnolls are always handy monsters to have around - whether as generic beastmen or to fulfil specific roles in games such as D&D, Frostgrave and Elf, Knyghte, Pyke and Sworde. 

They are ultimately derived from Lord Dunsany's story How Nuth Would Have Practised His Art Upon the Gnoles. The first edition of D&D makes this plain, along with the suggestion that the creatures were part gnomes and part troll - surely the inspiration for the original Citadel gnolls, which were eventually renamed "great goblins".

So why did they become hyena-men in later editions of D&D? I think it's just part of a general trend by which the "giant class" creatures - essentially a hierarchy of ever-more dangerous goblins - started to be differentiated through animal attributes. So kobolds became dog-men, goblins preserved the archetype of the Tolkien orc, orcs became pig-men, hobgoblins monkey-men or ape-men, gnolls hyena-men and bugbears bear-men. 

Interestingly, gnolls have preserved their animal identity through the various iterations of basic and advanced D&D as most of the others have not. Perhaps that's because hyena-men are just a more evocative concept - conjuring the ghouls of Arabic folklore as well a whole range of unsavoury habits. Hemingway summarises these nicely (if a little unfairly); his description is a great starting point for GMing gnolls:

"The hyena, hermaphroditic self-eating devourer of the dead, trailer of calving cows, ham-stringer, potential biter-off of your face at night while you slept, sad yowler, camp-follower, stinking, foul, with jaws that crack the bones the lion leaves, belly dragging, loping away on the brown plain."