From bulbapedia:
HeartGoldSoulSilverSwarmRoute 3
-Mudkipie
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You have to get an action replay and battle one of the elite four
It is possible to get the Pokemon Gulpin in Pokemon Platinum version by going to the Great Marsh. Gulpin is a poison type of Pokemon.
Gulpin evolves into Swalot at LV.26
Gulpin evolves into Swaloti at level 26 in Pokemon emerald
Gulpin is #316 in the national pokedex, and it is a Poison type Pokemon.