You're thinking of generation 4, Pokemon Ruby/Sapphire/Emerald is generation 3. In generation 4 they added a de-evoultion of pokemon like Snorlax.
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you cant munchlax is knew on the Pokemon diamond and pearl national dex.
Go to the GTS and if you have Munchlax in your Pokedex, send in a Pokemon and ask for a Munchlax.
Meoth, Munchlax
Breed a Snorlax
your rival has a munchlax when u vs Jupiter and mars in spear pillar