Legendary Pokémon Appearances

When you have cleared the game and return to areas, there is a small chance that you will encounter various Legendary Pokémon. When you encounter these Pokémon, you will get into a boss fight and will have to defeat them in order to both proceed and attempt to obtain the Pokémon. You will note the encounters by the fact that the springboard that sends you to the boss room will be shiny. These Pokémon take the place of the boss for that level that time
Most of these appearances are not restricted to certain levels but in fact certain types ofs across all levels. These appearances are very rare, appearing for 5% of the boss battles, and may require numerous visits before it occurs. Some Pokémon have pre-requisites to appear, however all require you to defeat the Dark Rust and finish the story. The Pokémon are as follows

No. Pic Name Type Area Found
#150
Mewtwo All Factorys
After Completion of Battle Royale EX-6
#249
Lugia All Beachs
#250
Ho-Oh 3-2 Volcanic Slope - Autumnwood
3-3 Firebreathing Mountain - Autumnwood
#380
Latias All Meadows
#381
Latios All Meadows
#382
Kyogre All Beachs
#383
Groudon All Magmas
#384
Rayquaza All Towers
#386
Deoxys All Factorys
#483
Dialga All Caves
#484
Palkia All Lakes
#485
Heatran All Magmas
#486
Regigigas All Deserts
#487
Giratina All Horrors
#487
Giratina All Horrors
#488
Cresselia All Treetops
#491
Darkrai All Horrors
#493
Arceus All Towers
After completion of Team Battle EX-6
#638
Cobalion All Caves After defeating Terrakion & Virizion
#639
Terrakion All Canyons
#640
Virizion All Forests
#641
Tornadus 3-2 Volcanic Slope - Autumnwood
Requires 3 Encounters Before Battle
#642
Thundurus 3-3 Firebreathing Mountain - Autumnwood
Requires 3 Encounters Before Battle
#643
Reshiram All Caves
#644
Zekrom All Caves
#645
Landorus 3-2 Volcanic Slope - Autumnwood
Requires defeating both Tornadus and Thundurus
#646
Kyurem All Glaciers