Unless you have been hiding in a cave, you would know that the GSC Starter Pokémon Are Catchable in this game. Chikorita, Cyndaquil and Totodile
According to each one of their dex entries, these Pokémon are Found:
"Bonus Appearence on the Ruby Board"
That means that it is found basically randomly and rarely on the Ruby Board. It is quite rare that you find them, but they are found in Catch 'em Mode...so be prepared. For example I found Totodile in a cave.
Once these Pokémon have been found, capturing them is done as Normal. However unfortunately you cannot evolve these Pokémon. So That's all they're really there for...looks and to take the Dex up to 205