Our Druid has recently gotten access to the polymorph spell and has started polymorphing the opponents I so lovingly crafted for our (well, mostly my) enjoyment. (So far, he has been turning them into rabbits, not yet killer whales.)
Now, the canonical response to a polymorph is to reduce the polymorphed creature to 0 HPs, upon which it will transform back. Or to attempt to break the spellcaster's concentration.
However, polymorph is a 4th level spell, not a cantrip. It does not seem all that likely the target's allies know the spell, can connect the dots when their friend suddenly turns into a bunny, and start to attack the bunny or the spellcaster. It seems more likely that they will blink upon seeing their friend transformed into a bunny, shrug, write him off, and continue battling the PCs as before, which would essentially take the target out of the fight.
I have been thinking about allowing the target's allies an Arcana check each, against some reasonable DC (e.g., 14, which is 10 plus the spell level of polymorph). On a success, they recall hearing about the Polymorph spell and the two possible remedies. On a failure, they don't. In which case the target will remain polymorphed until it gets hit by a stray fireball or some such. I might even allow the target itself an Arcana check, and on a success, it will try reducing its own HPs. Well, this might at least make for some fun.
It looks to me like polymorph might be a bit overpowered against opponents who no have legendary resistance, and whose allies only consist of non-spellcasters because they will likely not know what steps to take to break the effect. Is there a better way to role-play what happens to a polymorphed enemy?
EDIT toTo address the concern about this being opinion-based: I fear that if adversaries often do not know how to break the polymorph spell, this will make combat more boring, since the PCs will simply open by polymorphing what (they think) is the most dangerous enemy, taking him out of the fight entirely. Also, it's asymmetric, since the PCs know how to deal with someone polymorphing one of them. What techniques could I as the DM use to make sure battles stay interesting, possibly even fun?