The Poltergeists (The Other Side) are a group of spirits from various backgrounds - gangsters, sailors, martial artists - who serve other ghosts in unsavory operations.
Fred procured a group of Poltergeists to take out the Ghostbusters for interfering in his and his bosses' business. Fred lured Egon, Ray, and Winston outside the Firehouse where the Poltergeists gunned them down. However, the Ghostbusters proved themselves a threat even in Purgatory. The Bosses dispatched Fred and a legion of Poltergeists to destroy the team once and for all. They were defeated and appeared to have fled back to the real world.
These Poltergeists have the ability to interact with the physical world, i.e. use guns.