The slayers had never encountered such a creature before as the one they now found themselves with in mass numbers.
They were called Moonfish - a species native to a different party of the galaxy than the one they were currently in. They derived their name from being mostly active at night or darker enclosed spaces; when hunting their prey they would swoop out of the air to grab them, bring them up to a particular height, and then drop them down below to allow gravity to help prepare their dinner.
Not to mention they traveled in packs of no less than 50 other Moonfish. They were a vicious species that had become overpopulated on their home world and had driven most of the other native species to extinction. That's when War and his crew were assigned to isolate the parasitic species and wipe them from existence.