The Legend of Ninetales was a legend, told by Whiscash. The player was once suspected to be the human in the legend and needs to run away, until Ninetales came and confirmed that s/he is not. It is later revealed that Gengar was instead the human.
A greedy Trainer entered a sacred dungeon that contains many rare Pokémon. The Trainer and his partner Gardevoir encountered a Ninetales and the trainer grabbed one of its tails. Ninetales casted a curse on the trainer, but Gardevoir absorbed the curse and sacrificed herself to protect the trainer, doing her responsibility of protecting her trainer. Ninetales took pity on Gardevoir and asked if the trainer wants to save her, but the trainer had already fled, scared of the curse, leaving Gardevoir behind. Ninetales then becomes angry with the trainer's behavior, and said that the human shall turn into a Pokemon one day and bring on many natual disasters when it happens, landing the curse of Ninetales on the trainer.
Whiscash's telling of the story
"There once lived a Pokémon by the name of Ninetales.
Now, Ninetales had many tails, all of them imbued with psychic power...
It was said that anyone so foolish as to touch a tail would be cursed for a thousand years.
But there was someone so foolish as to grab a tail.
And it was a human.
As one might expect, the human that grabbed the tail was subjected to a thousand-year curse.
However, just as the curse was cast, a Pokémon named Gardevoir shielded the human... And sacrificed herself to absorb the curse.
Taking pity on Gardevoir, Ninetales asked the human this... "Do you wish to save Gardevoir?" It asked...
But the human had already abandoned Gardevoir and fled.
Ninetales became disillusioned with the human... And it made this prediction...
"That human will one day be reborn as a Pokémon..."
"And when the human becomes a Pokémon..."
"The world's balance will be upset..."