Ninetails is the evolved form of Vulpix, a Fire-type Pokemon. While Vulpix is available in Pokemon Pearl, it is only found in the Grand Underground. This means you'll need to catch a Vulpix and then evolve it into a Ninetails by leveling it up.
Here's how to catch a Vulpix and evolve it in Pokemon Pearl:
1. Explore the Grand Underground: The Grand Underground is accessible after you get the Explorer Kit from the Pokémon Trainer Club in Veilstone City. You'll need to dig deep into the Grand Underground to find Vulpix.
2. Look for a Fire-type area: Vulpix is more likely to appear in areas with a fire-type theme, like the Magma Cavern.
3. Use a PokéRadar: The PokéRadar can help you find chains of Pokémon, making it easier to find Vulpix. You can obtain the PokéRadar from the Pokémon Trainer Club after completing the Sinnoh Dex.
4. Catch Vulpix: Once you find Vulpix, use your best catching tactics. Remember to weaken it first and use a Poké Ball that's effective against Fire-type Pokémon (like a Dusk Ball or a Timer Ball).
5. Level up Vulpix to evolve it: Vulpix evolves into Ninetails at level 36.
Remember that finding a Vulpix in the Grand Underground can take some time and effort. Be patient and keep exploring!