Well, it's definitely not true, but I too have wondered whether that episode referenced any kind of existing mythology or whether it was all pulled from whole cloth. I seem to remember an interview or something saying that during the first season, the producers tried to incorporate legends and superstitions into the plots, and then realized that they didn't have to and the show was better without them.
I just searched for Maasai myths about hyenas, and general African ones, and didn't find anything about them calling their victims by name. Doesn't mean it was never believed, but if Joss & Co. probably didn't find it from a highly reliable source. The closest thing I found was that werehyenas were/are apparently a thing.
On the subject of "The Pack" and its bad naturalism (which is a subject I love), here's a rundown of the errors about hyenas that I listed for another discussion elsewhere:
Kairos's Official List of Hyena Facts Showing That the Writers Totally Did Not Do the Research
1.) Hyenas are matriarchal; i.e., males are submissive to females. If Xander recognized Buffy as a potential mate, he should have also recognized her as an alpha, and accordingly grovelled for her attention - if he even thought he had that much status.
2.) Hyenas eat anything. They'd tear apart a piglet, sure, but they also wouldn't turn down a croissant or a hotdog.
3.) Forming a group and being together wouldn't at all make them stupider. Hyenas have a complex, sophisticated social hierarchy, and are altogether intelligent creatures who cooperatively use their numbers as their greatest advantage.
4.) They prey on the weak, sure, but that's true of all predators aside from man (and vampire).
5.) The zookeeper claims that they're dealing with a "very rare breed". I'm pretty sure there's a reason he didn't specify which one - wild animals are divided into subspecies, not breeds, and all evidence points to our culprits being the most common type, the spotted hyena.
6.) And here's my favorite: check out the documentary footage that Willow's watching twice in the episode. The very first shot is of a spotted hyena. Everything else is African Wild Dogs - not even in the same family!