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Really good question! Personally, I believe that they can manifest themselves in all sorts of ways, but I'm not sure if that is biblical.
Demons are spirits and are constantly searching for bodies to possess or to dwell in. Angels are not spirits and do not possess bodies but they can materialize producing a human body and show themselves as people.
It is my opinion demons can manifest in human form. I have experienced this situation. I commanded it to leave in Jesus name it did. If you go to godsbarnchurch.com under origin of church. This story is told about this instance with two photos of a powerful demon transitioning to a human form. No doubt on this one. The experience left me with the understanding of our Lord and savior is indeed real and available to us.