Mecha-King Ghidorah appears in seasons 5 and 9.
Robots Battle Royale
Mecha-King Ghidorah was sent to an arena where he , Cyber-Zilla , Mechagodzilla , Super-Mechagodzilla , Kiryu , M.O.G.U.E.R.A , Moguera, Mechaini-Kong and Black Mechagodzilla would fight. The teleporter Dr.Who (Not the one in America) shouted "3 2 1 Fight!". The monsters then fight. Mecha-King Ghidorah oblitarated M.O.G.U.E.R.A , Cyber-Zilla and Mechaini-Kong with the gravity beams. Dr.Who shouted "No Mechaini-Kong my precious robot". Then Kiryu bit Mecha-King Ghidorah's shoulder. Causing the Cyborg to roar in pain. Then he defeated the other robots then left to live with the humans.
Mecha-Gidorah vs Bagan
Mecha-King Ghidorah finds Bagan attacking his home. Bagan enraged Mecha-Ghidorah. Then Mecha-King Ghidorah used grappling hooks and sent bagan to an active volcano and dropped him into the lava.
Bagan is Mecha-King Ghidorah's only oppenent who is not a robot or cyborg.