TL;DR A barbarian suspends any active rage and may not enter a rage while polymorphed via a polymorph spell.
The other evening, in running the D&D 5E adventure Tomb of Annihilation, the players brought forth a raging King Kong. The warlock polymorphed the raging barbarian into a giant ape. We went with the rage continuing for the barbarian; after all we allowed the druid/barbarian to rage in wild shape. But that polymorph ruling festered as I thought through the long-term ramifications - a huge pile of hit points that burn at a far slower rate.
The rules are a bit murky. Rage requires a bonus action to activate, so while polymorphed a character could not start nor deliberately end a Rage. But what about polymorphing someone already raging? Is it important that Enraged is not a game condition, akin to poisoned, restrained, etc?
Let’s look D&D team responses on Twitter.
From Jeremy Crawford, lead rules designer:
Polymorph replaces your game statistics, including class features, with those of the beast. If you’re a barbarian, you lose Rage.
Mike Mearls, co-creator of 5e responded Yes to the following question:
Barbarian rages, get Polymorphed into Giant Ape. Does the character keep the rage on them as a Giant Ape?
Conflicting answers from two reliable sources. Jeremy Crawford’s response provides deeper transparency into the reasoning, so I’m inclined to lean towards that answer.
I also want to look towards why barbarian/druids can rage and wild shape. For reference, here is the relevant druid wild shape ability.
Druid Wild Shape You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so. However, you can’t use any of your special senses, such as darkvision, unless your new form also has that sense.
Where druid wild shape grants explicit permission to keep the benefit of any features, polymorph does not. Polymorph instead limits the actions you can perform.
My refinement to polymorph is:
The creature loses the benefits of any features from class, race, or other sources and instead can perform actions according to the nature of its new form. It can’t speak, cast spells, or take any other action that requires hands or speech.