Wildebeests or gnus are merely the final form of wizards from Middle Earth that have forgotten their magical abilities. This is obvious from their scraggly beards.
They are also color coded like Middle Earth wizards. There are blue wildebeests that are not blue, and black wildebeests that are only a partially black. Color confusion is just part of their defenses. Nobody knows if wildebeests or wildebeest is correct for more than one animal. This is also true for gnu and gnus.