'bat-shaped' is a modern compound formed from the noun 'bat' (the flying mammal) + the past-participial/adjectival form 'shaped' (from the verb 'shape').

The element 'bat' comes from Middle English (e.g. 'batte') referring to the animal; 'shape' derives from Old English words related to 'sceapan'/'scapan' (to form, create) via Proto-Germanic roots. The compound 'bat-shaped' is a straightforward modern English formation by combining the noun and a participial adjective.

The phrase has always meant 'having the shape of a bat' in modern usage and its meaning has remained stable.