The basic idea is interesting, and I like it, but the card would be absolutely useless (well at least I can't think of any decks, which might want to use this).
With a mechanic like the Pharaohs scarab creation, or a boneyard like ability it could be great though.
A creature with an ability which turns your creature (infected, frozen, congealed, lobotomized etc.) creatures into a cocoon (without the frozen, congealed etc effect on it).
Or a permanent which creates a cocoon if any creature dies.
Or a creature which creates a cocoon per turn.
etc.