I think this anon is the most accurate.
It's meant to represent a man hanging upside down without his head, and with his hands cut off (presumably so he can't escape). Given the disturbing inhumanity of the form, I think it's fair to conclude it's a collection of spirits or something rather than one really pissed off and powerful one. I believe it was unleashed with the help of Ganondorf rather than made by Ganondorf. It existed, sealed, in those deep dark ruins of Hyrule's past.