The D&D 5E Monster Manual's entry for mimics it mentions that it "reverts to its true form when it dies."
I'm assuming a chest isn't its true form. What actually is its true form?
According to the MM, page 220, its true form is amorphous. That means that it is without form in its true state. You could describe it as a blob.
Shapechanger. The mimic can use its action to polymorph into an object or back into its true, amorphous form.
without a clearly defined shape or form.
I always envisioned the mimic as some sort of slime in its natural form.
The Monster Manual states that the Mimic returns to it's "amorphous form" when slain, and the definition of amorphous is "without a clearly defined shape or form."
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