Appart from grass-types, other Pokemons can well counter or check Hippo: Misdreavus can take hits from Hippo really easily except rare Crunch and cripple it with Will-o-wisp or simply use Shadow Ball. Although it can't set up Nasty Plot because of Whirlwind. Despite it isn't really good, Bronzor doesn't take something signifiant from Hippopotas and can cripple it with Toxic and/or set up Stealth Rock. Natu is another way to deal with Hippo. It can come on any move bar Crunch which isn't really common, Magic Bounce Stealh Rock and can cripple it with Toxic. Staryu can come on Hippo and Spin Stealth Rock and can OHKO Hippo with Hydro Pump. However, it has to be carrefull because it takes a lot from Earthquake.