late Middle English: perhaps from a variant of Old French chatepelose, literally 'hairy cat', influenced by obsolete piller 'ravager'. The association with ‘cat’ is found in other languages, e.g. Swiss German Teufelskatz (literally 'devil's cat'), Lombard gatta (literally 'cat'). Compare with French chaton, English catkin, which resembles a hairy caterpillar
The ending of caterpillar is spelled -ar.