It is in the Great Marsh, you can get in there once you defeat the town's gym leader.
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No. You can only get the HM for defog in Pokemon Diamond, Pokemon Pearl, and Pokemon Platinum.
youdon't defog is a hm move that is in Pokemon diamond, pearl, and platinum
you cant get defog in the earlier versions of pokémon, only in diamond and pearl
teach a flying Pokemon defog and use it
There is no HM Defog on Ruby, it came available in the 4th generation which is Diamond, or Pearl.