The legendaries that may be requested as customs:

Quote:

Articuno
Celebi
Entei
Giratina
Heatran
Ho-Oh
Lugia
Moltres
Raikou
Registeel
Regirock
Regice
Shaymin
Suicune
Zapdos


Pokemon that may NOT be requested as customs for various reasons:

Quote:
Arceus
Azelf
Cresselia
Darkrai
Deoxys
Dialga
Ditto
Feebas
Groudon
Jirachi
Kyogre
Latias
Latios
Manaphy
Mesprit
Mew
Mewtwo
Milotic
Minun
Palkia
Plusle
Rayquaza
Regigigas
Uxie