If the druid is a player, this does not work
Ironically, it's a really simple reason: A creature affected by Polymorph must have a CR or level greater than or equal to the Challenge Rating of the creature they're transforming into; and player characters don't have Challenge Ratings. And since they don't have the same stats as whatever they've transformed into, it's not appropriate to just paste the form's CR onto them.
So a Druid that has taken the shape of a Black Bear, lacking a Challenge Rating, would be ineligible to be the type of animal that a creature targeted by Polymorph could turn into.
If the druid is an NPC, it gets tricky
The primary restriction on Polymorph is "a beast with CR equal or lower than the target's", without the same stipulation found in Wild Shape that it has to be a beast the caster has seen before. Because the Druid NPC has a CR—and presumably it was calculated with the alternate shape forms it might take taken into consideration—the real questions are whether
- The Druid NPC counts as a beast while in that form, and
- The Polymorph spell allows the target to be polymorphed into a "specific" version of a beast, and not just the Platonic Ideal of a beast, and
- The Shape Changed Druid NPC represents a specific version of a beast, and not their Platonic Ideal
For the first question, I think the answer is yes, they do indeed count as a beast, given answers to questions that rely on similar logic.
For the second question, I think the answer is also yes, it allows the target to be turned into a specific animal. There's no text contradicting this, and even specific, non-demicanonical beasts found in Adventure Modules have Challenge Rating associated with them, and generally are valid targets for Wild Shape and Polymorph.
So there remains the last question: does a Shape Changed Druid NPC represent a specific version of the beast whose form they've taken, or do they represent a generic version of the beast whose form they've taken? And I think the answer here is also yes: they represent a specific version of the beast. They are a beast with the same stats as the form they've taken, except with (presumably) higher mental stats.
So as far as I'm concerned, it is legal to Polymorph a creature into a Shape-changed or Wild Shaped [NPC] Druid, and doing so will result in the polymorphed creature inheriting the mental stats of the druid.
At my table, because of this ruling, I think I would then extend that to allow this to be valid if the Druid is a Player Character; but like I said, I do not believe that is Rules-as-Written valid.
