They are in the maniac tunnel.
You can only find Darkrai through a Pokemon Event.
route 214 it may be rare catch
battle island
hippotas is on iron island
You find hippopotas (note I spelled it right) in the maniac dude's cave
underground. look for an explorers pack in Eterna city.
No
uxie,hippotas,hippodon and snorlax(yes he is in platinum).
you dont you pea brain
You have to have seen every Pokemon in the regular dex.
you go in the cave on route 214. walk around til you find one.(there are a lot of geodudes.)
You can ask friend for a hipodon mate it with a ditto