bulbapedia is your best bet. to find more info
which have you decided on?
some easy ones would be suicune (all of the crystal stuff between you and eusine) and the 4th movie
arceus (D, P, Plt plates)
the 3 birds and lugia (2nd movie and silver text)
mewtwo (cinnibar island mansion diaries and 1st movie, and mewtwo returns)
dialga, palkia, azelf, mesprit, uxie, girantina (D, P, Plt)
and mabey celebi (4th movie and the orange islands anime (GS ball stuff)
hey found this for ya....
http://bulbapedia.bulbagarden.net/wiki/Category:Pok%C3%A9mon_myths