You'll need to trade for it.
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you can't, its only possible to catch him in Pokemon silver and gold on route 6
with a pokeball
You can not catch it in leafgreen,but in firered you can.
You'll need to trade for it or use GameShark.
no Mareep can only be found in silver and gold so it must be traded. You can get Mareep in Routes 32, 42 and 43 in the other games so..