Can a Druid Wild Shape into a Swarm or "Giant"?

Question

The Players handbook has a list of creatures in the back which are designed for use by the players. Various tweets and materials have said that the players handbook has creatures that players need to create characters. However I noticed in the DM's Basic PDF that there are many creatures which are variants on the "basic animals" listed in the Players handbook.

These variants take on the form of swarm of [animal] or Giant [animal]. Some of the swarms and giant versions of the various animals are of a CR less than 1.

For example, we have in the Player's handbook the "Rat", which is a CR0. In the DM Basic booklet we have Rat, which is CR 0 but also Swarm of Rats, CR 1/4 and Giant Rat CR 1/8

Is a druid allowed to wild shape into the Swarm of Rats, Rat, and Giant Rat, or only the basic unmodified creature listed in the Players Handbook (i.e. Rat)

Answer 1

Note the uses of the word Beasts in the description of what Wild Shape does.

Beasts is a defined term; from page 2 of the Dungeon Master Basic Rules:

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.

You can Wild Shape into existing "Giant" animals

For Giant (x), if the creature meet the requirements of the Beast Shape table on page 66 and the Challenge Rating requirements on the same page, then the answer is yes, you can transform into a giant beast.

For example, a Giant Scorpion is described as

Large beast, unaligned

So it meets the Beast requirements, but its CR of 3 exceeds the maximum challenge rating of 1 for an 8th level druid. So you can't Wild Shape into a Giant Scorpion.

However, a Giant Poisonous Snake is CR ¼, so you can Wild Shape into this creature if you seen it before and are at least 2nd level.

You can't Wild Shape into swarms

As for a swarm, the answer is no. This conclusion hinges on the wording of both Wild Shape and the Swarm creature.

Wild Shape (PHB page 66) opens with

Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.

The plain reading means the druid character can shape into a single beast.

The swarm descriptions are formatted like this:

(swarm size) swarm of (creature size) beasts, (alignment)

For example a Swarm of Poisonous Snakes (DM's Basic Rules, page 46) is

Medium swarm of Tiny beasts, unaligned

So it clear that the "creature" is only treated as a single entity for the purpose of combat. It is really comprised of many creatures, whose action in concert gives the swarm its power.

My opinion is that they forgot to define or haven't finalized the definition of a swarm, which is why it doesn't appear in the list of standard creature types on page 2 of the 5e Dungeon Master's Basic Rules.

Answer 2

Rules designer Jeremy Crawford answered the "swarm" portion of this question in an unofficial tweet from January 2016:

Wild Shape lets you transform into a single beast. A swarm is a collection of beasts, not one.

Answer 3

Swarms are hive minds I say Yes you can

The Nature of Swarms

The swarms presented here aren't ordinary or benign assemblies of little creatures. They form as a result of some sinister or unwholesome influence. A vampire can summon swarms of bats and rats from the darkest corners of the night, while the very presence of a mummy lord can cause scarab beetles to boil up from the sand-filled depths of its tomb. A hag might have the power to turn swarms of ravens against her enemies, while a yuan-ti abomination might have swarms of poisonous snakes slithering in its wake. Even druids can't charm these swarms, and their aggressiveness is borderline unnatural.