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.
Jolteon
GLACEON
unown
The ID For Pokemon Platinum(USA) CPUE d074d1b3
In Platinum dex' its Dusknoir in National dex' its Sunkern.
on route 214.the Pokemon is hippopotas.
Go down Veilstone city and if so you could go to the spring path it will be easier
Pokemon Number 169 in Platinum is Leafeon
Mr.mime
Pokemon number 151 is Manaphy
Unown
Snorunt
rotom
gengar
wormadam.
its golem
Glaceon