It would help to know how you obtained such caterpie.
Normal Caterpies evolve into Metapods at Lv 7 and into Butterfrees at Lv 10.
It's tough for me to say it, because I still don't understand how/why they do it, but if it's a Caterpie that you didn't catch in game but received, it was most likely hacked/ alterated so to evolve before its time.
Let me know if this is the case, I'm quite curios!
Hope I helped!