Quote Originally Posted by da newt View Post
Q105
G: Is the Echo a "creature," an "object," an "illusion," or something else?
Quote Originally Posted by Chronos View Post
105 A:
G: It's never specified to be any of those, but it has stats as if it's a creature. It might also be an illusion: There's no rule that specifies that something can't be both a creature and an illusion.
R105-G For what it is worth, Jeremy Crawford has ruled that the echo is an object.

Note that having hit points, an armor class and immunities is nothing new for an object, as seen in the PHB page 185.