"The Ecology of the Kopru" a few issues ago offered an alternative origin for Demogorgon - that he is the child of two mothers, the Elder Evil Shothotugg and a being called Prukal who the kopru believe personifies the sky.
Now, this may be extremely unlikely (how would Demogorgon then be a tanar'ri?), and I'm more inclined to believe Iggwilv than the savage and superstitious kopru in matters regarding the Abyss, but it's out there for those who are interested. I like the "two mothers" angle because it implies that Demogorgon was once two separate beings, perhaps welded together by the unnatural experiments of the obyriths.