After my group's last session, it was pointed out that Polymorph does not require the caster to have ever seen the creature they are attempting to transform the person into. This has spawned a conversation between our Wizard and Druid.
The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating).
There is, as far as I can tell, no other restrictions. It must be a beast, and it must be CR lower than or equal to the character's level. You need not have any knowledge of the creature, other than knowing it exists at all.
Druid's Wild Shape states:
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
So, the Wizard and Druid player in the party began thinking this over. Wild Shape only requires that the druid has seen the animal before, but Polymorph has no such restriction.
The DM has been enforcing the "must have seen it before" rule on the Druid's wild shapes, but now the druid is coming up on 9th level and will be able to shift into CR3 beasts. Issue being, he hasn't ever seen one.
As far as I can tell, there is nothing stopping the wizard from polymorphing a willing ally into a beast that the druid hasn't seen before, which would enable the druid to then wild shape into it as long as the level requirement is met. Is there any rule preventing this, or any other reason why it wouldn't work?