I just came across something I never heard of before except for a similarity in Charmed. That is why I find it to be rather odd indeed. This excerpt is taken from the Satanachia page at donaldtyson.com
"When a demon is of high authority, it appears in human form. Demons in animal forms are of lesser importance. Still less in rank are demons that come in the shapes of insects or plants. And last in importance are demons that appear as objects. Satanachia's shape suggests that his true rank is quite low, regardless of what is claimed for him."
On the home page of donaldtyson.com, it says this:
"This site is designed to dispel some of the misconceptions in the popular imagination regarding various aspects of what is generally called the supernatural."
At the bottom of the home page, it says the copyright stuff, but assuming that this was done by Tyson and he was being truthful about the human appearance spirits, where would this idea come from? I've never come across it before. Lots of spirits, including the animal looking goets, often look like humans to me.
Edit: Some have shown representational parts that aren't human, like wings and holding objects and stuff, but more often than not, they look like humans or human based ghosts to me.