The Animal ShapesAnimal Shapes spell allows the caster to transform "willing" creatures in range into an animal. It also says:
The transformation lasts for the Durationduration for each target, or until the target drops to 0 Hit Pointshit points or dies.
Would an affected creature be able to drop this form at will?
On one hand, the spell explicitly states only 2 events can break it (spellthe spell naturally ending, or the creature dropping to 0HP0 HP).
However, it could be argued the creature no longer meets the criteria to be transformed since they are no longer willing.
The spell requires concentration, which also lends credibility to the idea that the willingness check may be a continuous thing.