Perhaps you are thinking of Rapidash, a Fire-type Pokémon that is known for its resemblance to a horse. You can find Rapidash in SoulSilver by evolving Ponyta, which can be found in:
* Route 36: This is the most common location to find Ponyta.
* Route 42: Ponyta is rarer here, but still possible to find.
Once you catch a Ponyta, you can evolve it into Rapidash by leveling it up to level 40.
Let me know if you have any other questions about Pokémon SoulSilver!