As with many Pokémon who have appeared since the first generation, some of Ninetales's Pokédex entries have been reused over the years. I've noted which ones below. (I can't really blame them; it's gotta be hard to come up with something new for the 10th Pokédex entry you have to write, and even then most of them are just paraphrased.)
Red, Blue, LeafGreen, Y: Very smart and very vengeful. Grabbing one of its many tails could result in a 1000-year curse.
Yellow: According to an enduring legend, 9 noble saints were united and reincarnated as this Pokémon.
Stadium: It is extremely vindictive. It is said to lay a 1,000-year long curse if its tail is pulled as a joke.
Gold, Stadium 2, HeartGold: Some legends claim that each of its nine tails has its own unique type of special mystic power.
Silver, SoulSilver: Its nine beautiful tails are filled with a wondrous energy that could keep it alive for 1,000 years.
Crystal: It is said to live a thousand years, and each of its tails is loaded with supernatural powers.
Ruby: Ninetales casts a sinister light from its bright red eyes to gain total control over its foe's mind. This Pokémon is said to live for a thousand years.
Sapphire: Legend has it that Ninetales came into being when nine wizards possessing sacred powers merged into one. This Pokémon is highly intelligent — it can understand human speech.
Emerald: It has long been said that each of the nine tails embody an enchanted power. A long-lived Ninetales will have fur that shines like gold.
FireRed, X: It has nine long tails and fur that gleams gold. It is said to live for 1,000 years.
Diamond, Pearl, Platinum, Black, White: Its nine tails are said to be imbued with a mystic power. It can live for a thousand years.
Black 2, White 2: Each of its nine tails is imbued with supernatural power, and it can live for a thousand years.
All of these entries agree that Ninetales's tails are mystical and are embued with mystical properties, and that if you mess with them, you're asking for trouble. What I find most interesting are the entries from Yellow and Sapphire stating that Ninetales was formed when nine wizards or saints merged into one. Obviously in the games it evolves from Vulpix, but Vulpix's entries don't say anything about this legend. Perhaps it was this merging that allowed Vulpix to be able to evolve in the first place?
The entries also cite Ninetales as a powerful, vengeful Pokémon, and call it mystical. I've always thought that this is evident in Ninetales's use of Ghost-, Dark-, and Psychic-type attacks. Ninetales can learn very few attacks that aren't typed Fire or Normal: the rest are all in those three types with only a couple of exceptions. Ninetales may be a pure Fire-type, but between its movepool and Pokédex entries, I can't help but wonder if they've ever thought of reclassing it as Fire/Ghost or Fire/Dark.
The Ruby and Sapphire entries are also particularly interesting. I mean, mind control? That's pretty hardcore, Ninetales. It's not surprising that Ninetales is said to be intelligent; I wouldn't want something with mind control powers running amok, thank you. Not to mention that Ninetales can live for a thousand years. It's not going anywhere anytime soon.
While Ninetales has always been my favorite Pokémon, and while I think it's gorgeous and if it was real I would totally want to cuddle it (I'm still eight years old at heart, shut up), I do think it's important to remember that everything the Pokédex says about Ninetales points to it being a creature you don't want to mess with. This is the kind of Pokémon you want to respect and hopefully it won't decide to slap a thousand year curse on you. ...Which, of course, makes it all the more fascinating to me.