There are certain pokemon that can only be caught when you trigger certain events, like castform.

Unlockable How to Unlock

Beldum See Steven's house after you beat the Elite four.
Kyogre Beat the Elite Four, and go to the sea lair.
Groudon Beat the Elite Four, and go to the land lair.
Treeko, Torchic, or Mudkip Help Proffesor Birch escape the wild pokemon
Ho-oh/Lugia Get the Mystic Ticket, and go to Navel Island
Deoxys Get the Aurara Ticket, and go to Birth Island
Mew Get the Old Sea Chart, and go to Faraway Island
Latios or Latias After beating the elite four, watch the T.V. downstairs in your house.
Latios or Latias Mix records with a Ruby or Sapphire with the Eon ticket, and go to Southern Island.
Castform Take out the rival team at the weather institute.
Rayquaza Get Magma and Aqua to get Kyogre and Groudon out of the cave, and go to Sky Pillar.
Chikorita, Cyndaquil or Tododile When you complete the 200 Pokemon of Hoenn. Professor Birch will call you on your Pokenav.