Mario & Sonic Guy
Old rivalries live on!
I've been hearing about Kyurem and an alternate form, but this is rather surprising, to say the least. The two forms do make me wonder about how they would be implemented onto Kyurem, if it's used as an Adventure Mode boss.
You can't argue with me about my fandom... Sonchu
Maybe for one example, you first face Kyurem in its basic form. After you defeat the basic form, one of the villains will resurrect it, and then give it either the Reshiram trophy, or the Zekrom trophy. This would result in you facing one of those two forms in a boss battle.
Now, whichever trophy that Kyurem gets would be determined by certain Adventure Mode events that occur before the Kyurem battle.
In The Arena, you'd only have to face one of the two forms, which is similar to how Galleom's battlefield can change with each Boss Battles run in Brawl.