The following Mega Mounts have been added or will be added after the next client update:
All Charizard-themed Megas
Mega Garchomp (plus its forms)
Mega Tyranitar (plus its forms)
Mega Rayquaza
Mega Metagross (plus its forms)
Mega Aerodactyl (not yet finished)
Mega Mewtwo X (not yet finished)
Mega Mewtwo Y (not yet finished)
Mega Salamence (not yet finished)
Mega Gengar (not yet finished)
Furthermore, Arceus, including all 36 variants, plus its Christmas-themed form, has been added.