More likely they will be artificial
Or if they have the technology to travel interstellar distances,
they may also be able to bioengineer themselves into any form
they desire.
So such ETI may not be "monsters" but something utterly bizarre
by our standards. But if you think of how little we know, human
ideas of bizarre don't require that much of a stretch. The same
with a species that could impersonate the god of a human religion
wouldn't need to be all that terribly advanced to fool enough people,
sad to say. Just smear something on a wall will do the trick.