Viricune

That's the fusion of Virizion and Suicune. They have similar features (such as height) and they are both the most elegant in their trios.

Here again the (enormous) list (I remember that these are only ideas):
 Volcanion- Heatran- Mega Metagross- Gigalith
Darkrai- Hoopa (new form) (and maybe also Dusknoir)
Mewtwo- Deoxys- Genesect (and maybe Mega Scizor)
Zekrom- Palkia
Lugia- Reshiram (and maybe Yveltal)
Giratina Original- Mega Rayquaza- Zygarde
Azelf- Jirachi
Primal Groudon- Yveltal
Shadow Lugia- Zekrom (or Yveltal)
Luxray- Arcanine- Pyroar
Mismagius- Lilligant- Florges
Reshiram- Xerneas- Cresselia
Ramphardos- Tyrantrum
Mega Altaria- Aurorus
Mega Gardevoir- Gothitelle 
Archeops- Noivern
Azumarill- Corsola
Scolipede- Mega Salamence
Dragonite- Goodra- Flygon (and maybe Mega Charizard X)
Sylveon- Victini
Clefable- Mega Audino- Chansey- Togekiss- Wigglypuff
Swanna- Mega Ampharos- Fufrou
Raikou- Liepard
Swanna- Mega Altaria- Reshiram- Whimsicott
Mega Latias- Cresselia- Lugia
Malamar- Froslass- Mega Mawile
Spritzee- Yamask
Meganium- Tropius- Aurorus- Scolipede
Rapidash- Zebstika- Keldeo
Castform- Solosis
Cinncino- Meowstic
Suicune- Zoroark
Cresselia- Ho-Oh- Xerneas
Mienshao- Ninetales- Mega Lopunny
Keldeo Resolute - Shaymin Sky
Suicune- Virizion

So... Enjoy!
Shiningfant