Our goblin druid posed a question today which made our DM bluescreen:

"If I turned into a giant toad, swallowed a Medium-sized creature whole, and then turned back, what happens?"

The goblin druid is a Small creature, and the giant toad is a Large creature. The creature being swallowed is Medium-sized.

This is essentially the inverse of this question, though not quite a duplicate because it's the eating creature doing the shapeshifting rather than the eaten one. Closely related to this question, with the primary difference being wildshape by a willing druid vs polymorph of an unwilling monster.

Lack of specific rules means DM have to decide.

Despite lacking an appropriate excerpt from the books, we can at least refer to the designer's intent in a close situation, as expressed by Jeremy Crawford on Twitter (Courtesy of Rubiksmoose):

Wild Shape can introduce wild situations. What happens when someone swallows a druid in a Tiny form? Is a druid fecund in beast form? The rules are intentionally silent on these corner cases, leaving adjudication to DMs. As always, I say go with what's best for your story. #DnD

It is also worth noting that the latest official sage advice compendium encourages the use of "Rules As Fun":

RAF. Regardless of what’s on the page or what the designers intended, D&D is meant to be fun, and the DM is the ringmaster at each game table. The best DMs shape the game on the fly to bring the most delight to their players.

So, let's try to anticipate the consequences of various adjudication options to keep the game fun, shall we?

1. If the creature is harmed, but the druid is not:
   
   Whether you inflict massive damage, or just decide the creature disappears to nothingness - this means the Giant Frog becomes a killing machine. A level 4 moon druid probably does not need that kind of help.




2. If the druid is harmed, but the creature is not:
   
   Then swallowing a creature becomes very risky. It removes the Giant Frog's main ability, and encourages the druid to stick to other combat forms. Bears and wolves. Yawn.




3. If both the druid, and the the creature are harmed (such as suggested here):
   
   Then swallowing a creature becomes a valid, sacrificial move - that may have its narrative interest. However, the DM should ensure it can't be exploited too easily: killing a BBEG with that method only to have the druid back on his feet with a healing kit should feel like cheating.




4. If neither the druid, nor the the creature is harmed:
   
   Game continues as usual, with many messy descriptions (if it is your thing) but few mechanical impacts.

Obviously there is plenty of nuances to be found between "harmed" and "unharmed", and every DM will choose whatever fits best. Option #4 seems the most safe and sensible option to me (I'd probably go for "both prone") - but #3 may lead to interesting situations if handled properly. This is only opinion though.

The more impactful the adjudication the DM has in mind, the more important it is that players be informed of the probable outcome beforehand (or that the characters be given an occasion to learn it through experiment). There are probably very few tables where the druid's bloody, unexpected death during a minor encounter would be welcomed as a "fun" surprise.

When a target of an effect ceases to be a valid target it is no longer affected

A giant toad can swallow a creature, your druid cannot - as per the effect, I mean. Obviously, the druid can swallow things that a goblin could swallow - things I don't want to think about.

As the druid transforms from giant toad to goblin the swallowing ends. Feel free to describe how this happens in as much nauseating detail as you like.

RAW - the only interpretation can be taken from the equipment section of the Wildshape description (emphasis mine):

You choose whether your Equipment falls to the ground in your space, merges into your new form, or is worn by it. Worn Equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of Equipment, based on the creature's shape and size. Your Equipment doesn't change size or shape to match the new form, and any Equipment that the new form can't wear must either fall to the ground or merge with it. Equipment that merges with the form has no effect until you leave the form.

Albeit, this is referencing equipment that is worn by the druid when shifting into a beast form. So for example, your gnome druid would not be able to keep wearing its leather armour - the toad form is too big to wear it. So it must either be dropped, or merge with the transformation.

This could work the same way in reverse. Upon transforming back into the gnome, the medium creature would either be merged with your PC, or dropped (i.e., regurgitated).

Again, this will probably have to be ruled by your DM.

The rules are "intentionally silent" on this issue

There are several abilities (magical or otherwise) that allows a creature to change shape or size in DnD. Examples include spells like Polymorph Enlarge/Reduce, and abilities like the Druid's Wild Shape. From a mighty Kraken to a lowly goblin, almost any creature in the game might suddenly find itself in a new form.

Placing general rules on what happens if such a creature has to change when there is not enough room to do so is rather dangerous. It could provide a way to defeat (nearly) any enemy, or otherwise upset the balance of the game. Perhaps that is why Jeremy Crawford issued the following statement on twitter (bold added) :

Wild Shape can introduce wild situations. What happens when someone swallows a druid in a Tiny form? Is a druid fecund in beast form? The rules are intentionally silent on these corner cases, leaving adjudication to DMs. As always, I say go with what's best for your story.

There are any number of related rules that could be seen to be relevant (such as what happens when a creature attempts to teleport into an area that is occupied, for example in the spell Dimension Door). But any of these attempts will be extrapolating rules into a case that the rules are meant to be "silent" on. According to the designers of the game, we will not find a RAW answer on this topic. A DM will have to decide.

I remember this question (or a similar one) popping up in a campaign I was in. I believe our DM said that though it's a DM call, he basically sees it as a logical question.

First, note that a toad swallows its food whole. That said, until it's digested, the meal is still in one huge piece. Also, how long took place between eating and transforming? Third, what kind of medium monster was it?

All in all he viewed that as what would actually happen, given everything in consideration, if your giant toad swallowed a deer, then transformed back to a goblin right away, chances are, his head would explode, but if the toad ate the deer and turned back an hour later, then he'd be more lenient to say it's digesting fine.

What's interesting is if your goblin turned into something able to eat a giant toad, then turned back, he'd have the transformation go off as you barf out the toad, resulting in your goblin looking like living toad poop.