Desna is one of the oldest gods. She only relatively recently (AKA when she discovered humanoids) took a humanoid shape as her own. What her form was before that is unknown. How she gained her knowledge of the Dark Tapestry is also unknown, but has something to do with the fact that she prefers outer space as her home (even though she also has a domain on Elysium) and that place is where the Dark Tapestry is located, and so she comes into conflict now and then with Great Old Ones and Outer Gods and the Dominion of the Black and whatever else is in the Dark Tapestry.
Cool, thanks for the info! Heh, somehow I continue to find Desna to be one of the most interesting gods despite her butterfly theme. ;)