The intrinsic property polymorphitis gives a 1% chance per turn of polymorphing into a random monster. You stay in the new form for 500 to 999 turns, unless your polymorphitis causes you to polymorph again earlier, or unless your new form is higher level than your original, in which case this time is reduced.
Only the ring of polymorph confers this property. Polymorphitis can be acquired permanently only by eating the ring. Like teleportitis, polymorphitis is not that desirable unless it can be controlled, or you are at a very low level and like to gamble.
Slash'EM Extended has playable Mould and Ungenomold races that start with uncurable polymorphitis, and sources of polymorph control have no effect on them. They don't suffer the risk of unsuccessful polymorphs though, and moulds have increased polymorph timeouts allowing them to stay polymorphed for a longer time while ungenomolds don't unpolymorph at all if their polymorph timer runs out.