What Process do Mimics go though when they Shapeshift?

Question

While a Mimic is in its true form, how does it polymorph into new objects? Does it naturally morph into them? Dissemble an existing one an build it around itself? or magically flash into the chosen object in an instant?

Answer 1

It's up to the DM.

The Monster Manual tells us that:

1. A mimic can change forms as an action, so it just takes a few seconds for it to change.
2. Once it has changed forms, a mimic is indistinguishable from the object it is imitating.

There's nothing to indicate it needs to consume an object in order to assume its shape, or even that in needs an object to "work from". How the change actually appears is totally up to the DM.

Answer 2

The rules say the following (Monster Manual, p.220):

The mimic can use its action to polymorph into an object or back into its true, amorphous form.

This means that it does not require another object to do this. However, mimic's Adhesive ability indicates that it could theoretically do this if it wanted to, but because of the wording of the Shapechanger ability it cannot have these items on it when it changes. As such, if a mimic wanted to coat itself in something it would have to change into the desired shape, grab whatever it wants to coat itself in and then move to the desired position. Though given its shapechanging ability it does not really need to do this, unless the object in question has some kind of effect the mimic cannot reproduce.