Nostalrius Begins - Quality wow vanilla realm (1.12)

Hey guys is this a bug or?

http://imgur.com/mgDTAuj

Technically, I suppose it is. You could argue that until the cocoon opens and you see a Butterfree emerge that it could be, or not be a bug. Have Metapods been studied intensely enough to know if other parasitic Pokemon would infest the shell and eat the creature from the inside out leaving an empty shell, and thus being devoid of a bug pokemon inside.