D&D 5E (2014) No RAW for this?

[Luke Haverland]

How to handle ghost possession of polymorphed and beast shaped PCs?

 

[jaelis]

Possesion incapacitates you, so polymorph spell would end. If you're thinking about wild shape, that doesn't end until you fall unconscious, hit zero hp, or die. So you'd stay in beast form.