The ability that returns a creature doesn't target anything. You can return a creature with shroud or protection from green, for example. You don't decide which creature to return until the ability resolves. If you control no other green creatures, you must return Stampeding Wildebeests itself.
You choose a creature and return it all during resolution of the triggered ability. This means that if you have two Wildebeests, there is no way to end up returning only one creature.