A Werehyena is a monster of folklore related to a real-life fear or hatred of hyenas by many indigenous tribes in Africa - who believed in supernatural creatures that superficially resembled hyenas but had numerous evil abilities which they used against their human enemies: they are similiar to the skin-walkers of Native American folklore except that unlike most werewolf-type creatures the Werehyena was not always requirred to be a human - in fact some tribes believed hyenas could take human form at will.
According to legend a Werehyena was capable of taking three main forms: that of a human, that of a large hyena or a hybrid of the two species - they could be solitary or work in packs and had a liking of calling out people's names at night in an attempt to lure them away from their friends and family, if a victim fell for this trick the Werehyena would proceed to devour them.
Evil magicians and witches were accused of being able to transform into these monsters at will and African superstition also blamed blacksmiths for supposed Werehyena attacks as many villages believed that blacksmiths were especially prone to becoming such creatures.
Unlike werewolves the Werehyena was not restrict in when he or she could appear but they prefered the shadow of night and when not stalking human prey they were said to engage in all manner of depravity including grave-robbing, cannibalism and piratical raids on villages and crops.
Werehyenas were undoubtedly the result of superstitious fear of the very real wild-dog known as hyenas coupled with prejudice against certain outsiders (blacksmiths were often Jewish, thus it may well of been a type of antisemitism): it was also likely inspired by the rich heritage of African religion that is still prevailent in today's world despite large-scale conversion to mainstream religions - perhaps the most striking example of this is in an African cult that deliberately dress up like hyenas and try to become hyenas via imitating the animal, members of this cult are often feared by others due to this behavior.
In Popular Culture
the concept of the Werehyena was explored in the Buffy episode "The Pack" - where hyena spirits possessed Xander Harris and a gang of thugs, this "Pack" began to devolve into predatory monsters as a result and all of them (save Xander) became cannibalistic as a result. The Primal cult that Doctor Weirick belonged to was also likely inspired by the real life African hyena cult.
Also the Gnolls in Warcraft and other fantasy games are often depicated as humanoid hyenas - though they are not technically a were-creature.