Official Mega Houndoom, Mega Tyranitar and Mega Garchomp sprites do not exist. (They shall be made eventually on these Forums, but until then, they are unavailable.) The Smogon sprites are from Smogon, so we cannot use them. Sorry, but denied.Pokemon to be Fused (Up to 4) :: Hydreigon, Mega Houndoom, Mega Tyranitar and Mega-Garchomp
Main Pokemon among the Pokemon to be Fused :: Hydreigon
Houndoom's horns should be on Hydreigon's head, while Tyranitar's chest thing should be fused into Hydreigon. Garchomp's arms should be fused as well. I'd like it if you could make the horns a bit more spiky though.