You can acquire an Ampharos in Pokemon Emerald by catching a Mareep in the Extended Safari Zone after beating the Elite Four. This area contains many Pokemon not seen normally in the Hoenn region. After catching this Mareep, you can level it up and at level 15 it will evolve into a Flaafy. Then the Flaafy will evolve to Ampharos at level 30.
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Since you can't get mareep in firered trade one from emerald.
Number 180 in Pokemon Emerald is Flaffy the evolvtion of mareep and the pre-evolvtion of ampharos.
You can't you must trade from Emerald, the Lost Cave is programmed to have mareeps but it's an event that Nintendo never had
I believe Ampharos is somewhat based off a Kangaroo shape figure.
You can get Ampharos by evolving Flappy which you can get at route 222 (with Pokeradar) but it's uncommon.