Barely-seen bloodbats
Bloodbats are a species of animal briefly encountered ✣ by Agatha, Violetta and Moloch while sheltering on a rafter in Castle Heterodyne. Very little information about them is offered, apart from the fact they take mass flight at periodic intervals and it's a very bad idea to be around when they do so. Their name may offer an indication as to why the latter is true.
According to a footnote in the novel Agatha H. and the Voice of the Castle, their scientific name is Desmodontinae Extremum Sanguine Comedenti, they are indeed blood-drinkers and they were deliberately imported into the Mechanicsburg area from "the Subterranean Kingdom" by The Black Heterodyne. They are preyed on by Venomous Rafter Toads.