Pokémon outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex, every day in a particular location, a "swarm of Pokémon" appears, which changes the probability of finding the given Pokémon from 0% to 40%. Outbreaks last only one day and are random events
Unfortunately, swarms are random events and there is no actual percentage as to what swarms will appear. Of course, if you really want a Pokemon that is unobtainable in that game, you can always trade from a previous game, which is probably the easier option.
Mawile is available in Diamond, Pearl and Platinum at Iron Island with Ruby inserted into the GBA slot, and it can be found at a number of places in Ruby itself.
Hope I helped. :)