Wearing a ring of polymorph gives you polymorphitis, which has a random chance of polymorphing you each turn. This ring is most of the time generated cursed, due to the fact that uncontrolled polymorphitis is really a pain. You can get system shock due to the ring.
If you have polymorph control, you may choose the form into which you polymorph. If you are inflicted with lycanthropy, the ring of polymorph will occasionally trigger a change into were or normal form.