Darkness Without Form: Secrets of the Mimic (2009)

From publisher blurb:

Most adventurers assume mimics are just wizard-created guardian monsters, and don't even think about them until a treasure chest sprouts arms and fangs. But mimics have a secret legacy--they were created by the aboleths to help dominate the surface world and enslave humanoids, and the well-known dungeon predators are merely the survivors of these experiments gone rogue.

In this book you'll find out more about the history of the mimics, the aboleth-symbiote "landwalker suit" that is the progenitor of all mimics, two kinds of humanoid-symbiote mimics, and more than a dozen new mimic abilities.