Leermus eggs can fly. Even though they haven't "hatched," they are very much alive creatures with minds (albeit underdeveloped) and wills of their own.
Unlike most bat creatures, Leermusses have the ability to see through their large, singular eye. Their way of seeing, however, is incredibly dangerous to both Arkians and many other creatures—their vision emits high-pulse X rays and radiation that's damaging to living cells. If you're being seen by a Leermus and can see its eye, it's too much exposure. Run for safety or get around a corner!
Adult Leermusses then acquire another ability: the ability to wield a magic staff. The Science and Research Center in Ark City provides no data and information on what Leermusses can do with their magic staffs presumably for Arkians' own safety.