Whistimmu is the ancient wraith term for the non-Spectre creatures than inhabit the Tempest. While there are loose categorizations for Whistimmu, any attempt to define them on more understandable terms is almost impossible, as they constantly develop and take on new forms, strengths, and weaknesses. One major category that does exist is Plasmics.
The origins of Whistimmu are unknown. Many wraiths have theories, of course. Some think the Whistimmu are formed from the leftover plasm of Drones that were pulled into the Tempest. Others think them the remains of "lesser" spirits, such as animals, and still others believe them to simply be manifested reflections of the Tempest's contained memories and dreams.