The Bat Wing Demons were entities that aided The Collector in his search for The Key. Due to the bizarre nature of the Land of Lost Objects, the Proton Packs had no effect on the entities, though Egon was able to improve that.
The Collector commands several lesser entities to aid him in his centuries long search for The Key, an artifact that will free him from the fortress in the Land of Lost Objects. When objects arrive in the Land of Lost Objects, a second type of entity, bat-winged entities sift through them with a magnifying lens. They were rather inhospitable when Slimer and the Ghostbusters arrived. No verbal communication was achieved and the demons continued searching in vain for The Key.