To get hippopotas in heartgold and soulsilver you need to go to safari zone and use special items.
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Hippopotas evolves into a Hippowdon.
Hippopotas evolves at Level 34.
You have to let it evolve from a Hippopotas, which you can find at the Ruin Maniac's Cave
Hippopotas, you can find it in Maniac Tunnel by Veilstone City.
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.