You have to get it in a swarm - Dawn/Lucas's little Sister will tell you who is swarming and what route if you speak to her after getting the national dex.
It's random so check each day.
Many Pokemon are possible, Edit this post if you know one I don't:
Larvitar
Beldum
Phanpy
Spinda
Natu
Voltorb
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No, you can not catch Phanpy in SoulSilver. It is a version exclusive to HeartGold.
You cannot catch Phanpy in Pokémon SoulSilver, it is only available in Pokémon HeartGold, Pokémon Ruby, Pokémon Sapphire, Pokémon Emerald, Pokémon Diamond, Pokémon Pearl and Pokémon Platinum.
You can't catch Phanpy or Donphan in Soul Silver; they're only in Heart Gold.
how do you catch a mewtwo on Pokemon platinum
you cannot catch the 3 regi's in Pokemon platinum you have to catch them in Pokemon emerald then transfer them to Pokemon platinum. thnx