Bats subsist on a diet of blood drained from other bloodborn creatures, most commonly other beastkin.  This tendency is a known cause of disease transmission, giving rise to tales that the bite of a bat can cause a sickness that leads men to froth at the mouth and lust after the blood of his kin. Defeating bats that threaten life and livestock can prove a challenge, however, as the bat bewilders its prey with ceaseless flittering, which makes attacking the beast an exercise in frustration. — Loremonger

From Encyclopædia Eorzea Volume I

These leathery-winged cloudkin make their homes in the shadowy recess of caves and tree hollows. Hanging dormant within their lair during the day, bats mainly venture forth in the hours of twilight. Bats possess incredibly acute hearing, and require nothing more than the echo of their own high-pitched cries to locate and catch insects in flight. Some species are known to feed on fruit, while others derive their sustenance from blood.^[1]