There are 8 islands. On some of them you can get some Johto Pokemon, but that's not all. On one of the islands you can get Ho-oh AND Lugia, and on another you can get DEOXYS, either the attack mode (Fire Red, ) or the defense mode (Leaf Green, ).
Also, the Legendary Dogs are back, but only one for each game, depending on your starter. With Squirtle, Raikou becomes available after you defeat the Elite Four. With Bulbasaur, Entei appears, and with Charmander, Suicune appears.
EDIT: Oh, and Moltres has migrated from Victory Road to Mt. Ember, which is on one of the islands. Just thought I'd add that.
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