Hitting a shambling horror with polymorph should not polymorph it into something else; instead it should rerandomize its stats and attacks.

This would be better if shambling horrors were randomized per monster rather than per species; it doesn’t make much sense to polymorph a horror on dungeon level 20 and a preexisting horror on dungeon level 17, or new horrors that haven’t even generated yet, get rerandomized.