by catch hippodon is and somewhere in sand it dig in sand and you need the Pokemon who have 3 faces
in the hippodon tempel.
If you want to get a Hippodon first see it on your Pokedex and have at least 1 Sinnoh GYM Badge and put a Pokemon up for a trade in the GTS. And hopefully some body will trade you for it.
In the Hippowdon Temple.
uxie,hippotas,hippodon and snorlax(yes he is in platinum).
Elite four Bertha has a Hippowdon. Just battle her and she'll send out a Hippowdon.
Espeon is the bottom room in the room with all the arrows that take u to another part of the room in hippodon temple.
When you're in Haruba Village you go up from the town and keep going up there will be a vine you need to cut, climb up there and it'll take you to the temple.
catch regice regirock and registeel than go to the hippodon temple and go through the suspicious hole in the springy grass room
you get the rest of the Pokemon and go to where the sliding floor is... the go to the very middle in the quicksand.. you need all the regis with you to open the door
You have to capture the Hippowdon. This may be challenging, since you go slower on the dark sand and the gusts of wind cause damage, but capturing Doduo will make it easier.
There are alot, these are the ones I know of : Gengar, Gardevoir, Dusclops, Dusknoir and Mismagius!