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?
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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.