you can get it in safari zone when you are able to lay down objects. place 28 rocky objects in the desert zone for it to spawn in the grass.
(may take a couple of days)
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To get hippopotas in heartgold and soulsilver you need to go to safari zone and use special items.
Hippopotas evolves into a Hippowdon.
Hippopotas evolves at Level 34.
in the maniacs cAVE
You must evolve a Hippopotas.