Is it possible for demons to materialize? Can they appear in either human or animal form? Though we know that demons can possess humans, is it possible for them to create some physical form and simply appear?
No Clear Biblical Evidence
As we look through the pages of Scripture we find no clear example of a demon materializing. Being disembodied spirits there is no indication that they have the power to create some human form in which to appear. This would be beyond their capabilities.
Need Bodies To Work Evil?
Furthermore, it seems that demons are powerless to work evil unless they are inhabiting a body. Jesus said.
But the unclean spirit, when he is gone out of the man, passes through waterless places, seeking rest, and finds it not. Then he said, I will return into my house from where I came out; and when he is come, he finds it empty, swept, and unoccupied (Matthew 12:43,44).
If a demon could simply materialize and cause havoc with humanity, then it wouldn't be constantly looking for some human form in which to inhabit. This is a further indication that demons cannot appear at will in some human-like or animal-like form.
When the demons were cast out of the men at Gadera, they did not wish to be bodiless - they would rather enter into the bodies of pigs.
And he besought Him much that He would not send them away out of the country. Now there was there on the mountain side a great herd of swine feeding. And they besought Him, saying, Send us into the swine, that we may enter into them (Mark 5:10-12).
Other Physical Forms
Although there is no evidence that demons can simply materialize, they can assume other than human forms.
Revelation 9 tells us that many demons were released from the pit.
And he opened the pit of the abyss; and there went up a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit. And out of the smoke came forth locusts upon the earth; and power was given them, as the scorpions of the earth have power (Revelation 9:2,3).
In this instance, their physical form was something like locusts, with the power to sting like scorpions.
Another Appearance
Later, in the Book of Revelation, we are told.
And I saw coming out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet, three unclean spirits, as it were frogs (Revelation 16:13).
The appearances of demons in these forms will happen only after the bound angels are released from the pit. This is still in the future. Some may argue that they are an example of demons having the capability to simply materialize. If this is the case, they did not materialize in a human form, but rather an animal-like form. Like so many other issues with respect to the topic of demons, we just do not know enough to make any solid conclusions.
Summary
The idea that demons can somehow materialize is not found in Scripture. To the contrary, the Bible speaks of these disembodied spirits going around trying to find human beings in which to possess. Demons do appear in animal forms in the Book of Revelation. Though it could be argued that this is a materialization of demons, there is just not enough information to make that conclusion.