Recently we had a player cast a Bestow Curse spell on a Polymorphed enemy in D&D 5e.
The question: Does the curse remain on the creature once Polymorph wears off, and/or they revert to their normal form?
If the answer is yes, Polymorph seems like an excellent spell to use in order to debuff an enemy due to the much lower saving throws in the polymorphed form.
Best Answer
Unless the caster's concentration is broken, the curse remains in effect1
While the creature is in its polymorphed form, the saving throw is based on the new form's mental stats (Wisdom).
Once it returns to its original form, as long as the caster's concentration has not been broken another save does not get rolled (except for option 3, see PHB p.218). The curse (general) remains in effect as long as the caster's concentration lasts, since polymorph's spell text does not indicate a "re-roll" of saves once reverting to original form.
In case 3, (the "do nothing each time you miss your Wisdom save" option) once the target returns to its normal form its saves should be with its normal Wisdom modifier. The text does not indicate that the curse ends if that save succeeds, only that the cursed creature does not get stuck "doing nothing" for that turn.
Debuffing
This does indeed benefit the curse's caster in the case where the target's normal wisdom is higher than in beast form. That said, the creature gets a save versus polymorph (Wisdom) in the first place, if your party is trying to change its shape. That means that the creature's first line of defense, its Wisdom save, is against the polymorph spell. If the creature had polymorphed itself, then that's not a problem.
1 Of course, if the spell is cast with a 5th level or higher slot, the concentration requirement is no longer applicable (PHB p. 218)