There are two types of Pichu. Normal and Spiky-eared. Spiky-eared Pichu is a special event Time-Traveling Pichu, so yes, it's in a category of it's own. It cannot evolve, but that doesn't end the Pikachu species. There are other female Pichus that can evolve but don't have spiky ears. Even if it was the last female and couldn't evolve, the remaining male Pikachus/Raichus can always breed with Ditto, with a chance of hatching a new female.
May this quench your curiosity.