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:
- Ankylosaurus
- Giant Scorpion
- Killer Whale
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:
OTHER ANIMALS
A book of this size can’t contain statistics for every animal inhabiting your D&D campaign world. However, you can use the stat block of one animal to represent another easily enough. For example, you can use the panther statistics to represent a jaguar, the giant goat statistics to represent a buffalo, and the hawk statistics to represent a falcon.
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.
Polymorph (4th level) is limited in what it can turn things into.
From the spell text (second paragraph):
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).
A beast is a particular creature type. The creature type is listed in the stat block of a creature. Compare adult red dragon (dragon) with allosaurus(beast).
Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals. (Basic Rules, p. 110).
Other types include fiend, fey, dragon, aberration, celestial, construct, monstrosity, ooze, and humanoid. Unless the creature form chosen is a beast (allosaurus, brown bear, etc) the 4th level polymorph spell won't turn the PC into a dragon. As far as the game is concerned, a dragon is not a beast; a dragon is its own type of creature.
Dragons are large reptilian creatures of ancient origin and tremendous
power. True dragons, including the good metallic dragons and the evil
chromatic dragons, are highly intelligent and have innate magic. Also
in this category are creatures distantly related to true dragons, but
less powerful, less intelligent, and less magical, such as wyverns and
pseudodragons. (Basic Rules, p. 110).
The 9th level spell, True Polymorph, has no such restriction.
Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating).
Your friend may be confusing the two spells, or may not see the distinction between the two creature types. On a humorous note, @NathanS suggests that the most powerful choices for polymorph (4th level) are Godzilla (T-Rex, CR 8, beast) or King Kong (Giant Ape, CR 7, beast). How does the character, or the player, feel about being polymorphed into one of those magnificent beasts? 😎 The PC would need to be level 8 or 7 (respectively) for that to work.
Best Answer
DM Fiat, but you probably cannot target the swarm.
Note that the very first part of the monster stat block says:
Per the Polymorph spell description, you can only polymorph a creature. So it should be ruled that the spell cannot target a swarm, or at a DM's discretion, that one beast in the swarm is changed with no effect to the rest (same AC, same hp, same everything).
As Rubiksmoose pointed out, the description text refer to the swarm as one unit. But it would be very clumsy to write out the sentences as individual beasts:
Related: Can a Druid Wild Shape into a Swarm or "Giant"?
Note that a Druid cannot Wild Shape into a swarm as they are multiple beasts. So it would be counter-intuitive that the reverse would work - that a swarm could polymorph into a single creature. Jeremy Crawford unofficially confirms this on Twitter here: