What Pokemon is number 122 in Pokemon platinum?

Answer 1

The Pokemon #122 in the Platinum Pokedex is the Ground type Pokemon known as Hippopotas. The Pokemon is quite rare. In the Soloceon Ruins, you can catch tons of different Unown. Well, in the route below Veilstone City, you can enter this tunnel. Depending on how many Unown you have caught (note that they MUST be different), the tunnel will grow. That increases the chance of finding a Hippopotas. Another option (though I am unsure if it's accurate) is to battle somebody in Iron Island, which can be accessed by sailing a ship in Canalave City.

Answer 2

Number 122 is Hippopotas.