It's possible you might be thinking of the "Incense" item. This item increases the chance of encountering a Pokémon of a specific type, which varies depending on the incense used. For example, the "Pure Incense" increases the chance of encountering a Psychic-type Pokémon.
Here's a list of the Incenses in Pokemon SoulSilver and their effects:
* Pure Incense: Increases chance of encountering Psychic-type Pokémon.
* Rock Incense: Increases chance of encountering Rock-type Pokémon.
* Wave Incense: Increases chance of encountering Water-type Pokémon.
* Odd Incense: Increases chance of encountering Ghost-type Pokémon.
If you're looking for a specific incense effect, let me know, and I'll be happy to help you find the right incense for your needs.