No Legendaries and no interactable Pokemon.
Pokemon In any game.
This is definitely one of the most rare Pokemon in any game at all,and this is yet to be compounded with the fact that Bouffalant has a 1% chance of showing up at all in a Hidden Grotto.
Hope I helped!
I don't believe there's such thing as the rarest Pokemon, but that Spiky Eared Pichu from HeartGold/SoulSilver is the rarest Pokemon I can think of.
It was an event Pokemon and it also cannot be found outside of HeartGold/Soulsilver because it cannot be traded.
I believe the rarest Pokemon is a 6 IV,shiny and perfect nature spiky eared Pikachu.