Monster Manual classification: Unknown
Boss: Red Named - Quest Boss
Habitat: The Haunted Halls of Eveningstar
Description: The doppleganger have no Type or Race on his examination screen.
Once he go low on health, he will provoke an explosion, and will hide between the other statues in the room. You can make some skill checks (no rolls) to try to find out which statue is him, and attack it. Guessing right on the first try will award an optional.
You can try as many times as you want.
Or you can just click and examine each statue. The Doppleganger is the one with the divergent CR.