You can find hippopotamus in Ruin Maniac cave on route 214.
Where To Find Pokemon Hippopotamus In Platinum Version?
Hippopotas or Hippowdon? Either way, you catch it in Ruin Maniac cave on route 214 after you catch every type of unknown at the Solaceon Ruins.
( PS. Don't use an escape rope in the Solaceon ruins or you won't be able to catch any Unknown there )
( PS. Don't use an escape rope in the Solaceon ruins or you won't be able to catch any Unknown there )