Even better: Psychic --> ???. I know, purists, there's never been a ??? type Pokémon before, but these are only suggestions. After all, ??? WOULD work, and as an added bonus, it is a symbol used to denote that something is UNKNOWN.
Palkia's Water type has mystified me. I don't really understand why Dialga is Steel either.
Listen Up Everyone:
All legendary trio pokemon have followed a pattern -
~Articuno (Ice), Zapdos (Electric), Moltres (Fire) ->
~Raikou (Electric, OLD), Entei (Fire, OLD), Suicune (Water, NEW) ->
~Regirock (Rock, NEW), Regice (Ice, OLD), Registeel (Steel, NEW) ->
~~But for Gen IV they switched it - instead of having the regular trio get the types they made the box trio get them~~ ->
~Dialga (Steel, OLD), Palkia (Water, OLD), Giratina (Ghost, NEW)
Basically they keep 2 types and add a new one after a generation:
1st gen: ICE, ELECTRIC, FIRE
2nd gen: WATER, ELECTRIC, FIRE (they kept elec and fire and added water)
3rd gen: ICE, ROCK, STEEL (they returned, ice and added two new ones)
4th gen: STEEL, WATER, GHOST (they kept steel from registeel, returned water from suicune and added ghost)
i thot everyone knew this.
http://www.pokecommunity.com/showthread.php?t=152830