Bats were small, flying mammals native to some parts of Gallowmere. They could also be summoned by powerful vampires.
In 1386, several bats could be found in Gallowmere. They were inhabiting caves such as the Crystal Caves, as well as abandoned buildings including the Scarecrow Fields' farm houses and the Asylum.
In 1886, the Count, an ancient vampire living in London, had the power to summon giant bats. Upon being defeated by Sir Daniel Fortesque, he also exploded into a flurry of them.
MediEvil (2019 remake)
They're vampire bats. Actually, they're just bats. But they bite. That's sort of vampirey.
Behind the scenes
In pre-release versions of MediEvil such as the MediEvil Rolling Demo, bats were present in The Ant Caves level. This is because the level did not originally have Dan shrinking down, but was an actual cave of human-sized ants.