New Stage - Pokemon Stadium 2
Mario's Down Special Move - F.L.U.D.D.
The previous game, Super Smash Bros. Melee, featured Pokémon Stadium, but it’s been upgraded for this game into Pokémon Stadium 2. This time around, you’ll see guest Pokémon appearing in the background. But as time passes, the terrain transforms into four different types:
"Electric," "Ground," "Flying," and "Ice." For these four transformations, we’ve chosen types that were not present in Melee.
Mario's Down Special Move is now F.L.U.D.D., but he can still use his Mario Tornado (his previous Down Special Move) though it's unknown how he can.
Source for Pokemon Stadium 2 (More Details and Pics Here)
Source for Mario's Down Special Move - F.L.U.D.D. (More Details and Pics Here)