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.
Polymorph states:
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?
Best Answer
This is unclear, and it's ultimately the DM's call. Beyond the general "what the DM says goes", it's not spelled out what exactly it means to have seen an animal. There's plenty of room for the DM to say that it's not seeing the shape that matters, but seeing a living animal with your mystical druidic connection, which won't work with a mere polymorphed form. (Or whatever similar reasoning.)
At first, I thought: it's kind of ... not so great ... for the DM to restrict class options in this way. But, on looking, the only three beasts at this CR from the basic rules (or for that matter, Monster Manual or Volo's Guide) are:
Many DMs don't allow dinosaurs ("there aren't any in this fantasy world"); Killer Whale is kind of limited even if you have gone whale-watching in the right environment; and Giant Scorpion also seems likely to be rather... environment-specific.
Personally as a DM, I'd let you play this trick with Giant Scorpion, mostly because you don't have any other options in the books and that's kind of lame. But, because of the specific nature of the three listed options, I can understand saying no, too. You might have to make do until next level, when you can shift into elementals.
Alternately, you might ask: "Hey, what beasts of around that power have I seen?" While there are a limited number of beasts listed in the rules, the Monster Manual says:
It's reasonable to assume that there exist CR 3 animals which you've seen but which aren't represented. As @enkryptor notes, the rule requires you to have seen the animal ever in your life, not just in this campaign. And, the Wild Shape rules don't say "a beast from the Monster Manual". So, particularly if you're not allowed any of the official listings, I think asking for something you can use is fair.
This could be as simple as "So, what is basically mechanically like an ankylosaurus or giant scorpion that does fit in this environment?" Or, there are several 3rd-party books of beasts (or monster books which contain some beasts) that might be worth looking into (search at DM's Guild, for example).. Or, you could ask the DM to create an option for you using the rules in the DMG, which gives typical hit points, AC, damage per round, etc. Maybe call it a "dire" version of something you've seen before, or pick a name which seems somewhere in power between a rhinoceros and an elephant.