Giant bat
From Ardrana
Giant bats are large versions of the carnivorous variety of the ordinary bat with 3-foot-long bodies and 5- to 6-foot-long wingspans. They dwell in dark caverns, usually underground, and depend on their sonar in flight to compensate for their poor eyesight.
Note: Giant bats originally appeared in this form in the 2nd Edition Monstrous Manual for the AD&D game from TSR. Their use here is for the purposes of providing context for the campaign only.