As far as I know you have to evolve it from Ponyta (Lv. 40)
catch ponyta north from the town with the place where they look after your Pokemon for you
transfer it from a GBA game (Like Saphire or Fire Red ect.) or trade.
You can catch one when you go to mt. silver from the indigo plateau
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Ponyta evolves into Rapidash at level 40.
The only fire Pokemon in the Sinnoh region are the Chimchar-Monferno-Infernape evolution chain or the Ponyta-Rapidash evolution chain. However, once you beat the Pokemon League you can get an Eevee and evolve it to Flareon.
You can find a Rapidash on: Route 22, Route 28, Mt. Silver
No, it is not a legendary Pokemon.
Rapidash is number #78 in the National Pokedex.