While today we recognize bats as mammals, Medieval thinkers categorized bats as birds. The Latin word for bat is vespertilio which is derived from vesper meaning evening. This is a reference to the time of day when bats emerge for their nocturnal activities. The medieval Cornish word for bat is Hihsommet (from ff. 7r-10r of Cotton MS. Vespasian A. XIV).
The bat, unlike other birds, is a flying quadruped, resembling a mouse. It has its name (vespertilio) from the time when it flies, after twilight. It flies about driven by precipitate motion, hangs from frgile branchs, and makes a sound like a squeak.
~ Isidore de Seville, Etymologiae c. 600 – 625 A.D.
This hairy bat
This happy little bat
This spread-eagle bat
Bat from the Aberdeen Bestiary – MS 24
The bat, a lowly animal, gets its name from vesper, the evening, when it emerges. It is a winged creature but also a four-footed one, and it has teeth, which you would not usually find in birds. It gives birth like a quadruped, not to eggs but to live young. It flies, but not on wings; it supports itself by making a rowing motion with its skin, and, suspended just as on wings, it darts around.
Translation from the Aberdeen Bestiary