In Pokémon Ruby, you can only catch Treecko by selecting it as your Starter Pokémon if you picked Torchic or Mudkip as your Starter Pokémon then you cannot find it or receive it in the game however you can find someone else locally that would be able to trade you Treecko additionally you could have Treecko from Pokémon Sapphire and you could trade it over to Pokémon Ruby in order for Pokémon Ruby to get Treecko.
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Ruby, Sapphire and Emerald.
There are two possible ways: Choose Treeko as a starter Pokemon Or By trading
raikou cannot be caught on Pokemon ruby
No.
You can't catch a sentret in Ruby version, but you can catch one in Emerald version.