Swallot

You have to transfer him from Ruby, Sapphire or Emerald. He is unavailable in the DS games otherwise.

To transfer, you need to have him in your party in Ruby/Sapphire/Emerald, have obtained the National PokeDex in Diamond/Pearl/Platinum and have the GBA game inserted into the GBA slot. When you boot up your game, the option to transfer Pokemon from said GBA game will be available. Do so, and then head to the Pal Park where you can catch the lot.

You need to catch a gulpin in Pokemon ruby/saphire/emerald then use pal park to transfer it onto your platinum game

You need to use your Gba games as followed: Pokemon Ruby/Sapphire etcc. You get the picture :P. Then go to pal park right where the battle frontier area is, but in the bottom left corner of that little island. Hope this helps and enjoy.

Gulpin is a daily Binocular pokemon and evolves into Swalot.

If you don't have any GBA games then just go into the Great Marsh and GULPIN will be there, but rarely.

Gulpin evolves into Swalot at Lv.26 and in Platinum version, Gulpin can be found in the Great Marsh.
Hope I helped you o^.^o
~Anna The Zoroark