So I just came to a 4am epiphany
From GOmens we know that demons possess people and angels can't right? But think about it, it would seem normal that Aziraphale already had a corpeal form back in eden but so did Crowley and since Adam and Eve were literally the first humans on earth where did Crowley get his body? Who could he have possessed if there wasn't anyone around?
This also provides an explanation why hastur, ligur and beelzebub's animal forms show thoroughly in their corpeal forms.
Crowley was given a body so his other form was already included in the machinery and other demons that possess people have their true forms show up outside their bodies - kinda like a ratatouille thing where remy was on linguini.
Ligur,hastur and beelzebub aren't the bodies we see, they're the animals on them
Thanks for coming to my ted talk