Does the lease say that nothing can be hung on the walls? Are deer mounts specifically noted as being prohibited while pictures, mirrors etc... are allowed? What about a bear skin? Can you hang it up? If the lease doesn't specifically mention deer mounts, I'd hang it and simply use spackling paste to patch the wall when I moved out. Renters have quite a few rights and I seriously doubt the landlord can enforce such a thing. In most places the landlord can't even enter the premises without the renters permission so how would they even know about it?
Now, if you don't want to risk a pissing contest you could get some decent lumber and make a stand to mount it on and put it in the corner of a room. It would have to have a pretty substantial base to be stable though. If it was me, I'd be looking around and I'd get out of that lease as soon as I could. Who needs aggravation like that anyway?