They can take on any form and enter anything.

Matthew 8:28 - 33 " And when he was come to the other side into the country of the Gadarenes, there met him two possessed with demons, coming forth out of the tombs, exceeding fierce, so that no man could pass by that way.And, behold, they cried out, saying, What have we to do with you, Jesus, you Son of God? are you come here to torment us before the time?Now there was afar off from them a herd of many swine feeding.And the demons besought him, saying, If thou cast us out, send us away into the herd of swine.And he said unto them, Go. And they came out, and went into the swine: and behold, the whole herd rushed down the steep into the sea, and perished in the waters.And they that kept them fled, and went their ways into the city, and told every thing, and what was befallen to the possessed of the devils."