Nope. That's a fan canon that's never been stated. Plus, it would be the opposite, Ganondorf being a variation of Demise. Anyways, that's not the case. There's no official material that ever presents Ganondorf as being a variation/reincarnation/son/clone/whatever of Demise. Demise and Ganondorf are two separate entities/characters. They're even more separate than the Zeldas, even though they are different people from the same lineage, but called the same, but at least these are incarnations of the goddess Hylia. And even then, we wouldn't say Hylia and the Zeldas are the same character, but thing is, Demise and Ganondorf don't even share the same connection as those two. The Zelda encyclopedia goes as far as to call Ganondorf a "champion" of Demise, along with Vaati and Malladus. Demise being the reason Ganondorf exists is onlyt because, as stated in the Zelda encyclopedia, Demise is the source of evil, and evil finds its way in its "champions". So saying it's a "variation" is missing the point entirely.
