Capture Kyurem!

You can capture Kyurem in the deepest part of the Giant Chasm. You better be prepared, this Pokemon is level 75 and has the move endeavor which can bring your hp down to Kyurem's! So make sure when your lowering its HP, It isn't too low. Bring many Dusk Balls for this Pokemon.

Kyurem lvl.75:Ability:Pressure:The Pokemon raises the foes PP usage Moveset:Glaciate/Dragon Pulse/Imprison/Endeavor
Stats:
HP:            *****
Attack:       *****
Defense:     ****
Sp.Attack:  ******
Sp.Defense:****
Speed:        ****

Capture Cobalion!/Terrakion!/Virizion!

You can capture Cobalion in Mistralton cave, in the Guidance Chamber, all the way in the back. After you capture or defeat it, the entrances that were blocked for Terrakion and Virizion become cleared. Terrakion is located in Victory road in the Trial Chamber, and Virizion is in Pinwheel Forest in the Rumination field. Good Pokeballs for Cobalion and Terrakion is Dusk Balls These Pokeballs work well for dark areas such as caves. As for Virizion, use Ultra Balls.

Cobalion lvl.42:Ability:Justified:Raises Attack if hit by a dark type move Moveset: Helping Hand/Retaliate/Iron Head/Sacred Sword
Stats:
HP:            ***
Attack:       ****
Defense:     *****
Sp.Attack:  ****
Sp.Defense:***
Speed:        *****

Terrakion lvl.42:Ability:Justified:Raises Attack if hit by a dark type move Moveset: Helping Hand/Retaliate/Rock Slide/Sacred Sword
 Stats:
HP:            ***
Attack:       *****
Defense:     ****
Sp.Attack:  ***
Sp.Defense:****
Speed:        *****

Virizion lvl. 42:Ability:Justified:Raises Attack if hit by a dark type move Moveset: Helping Hand/Retaliate/Giga Drain/Sacred Sword
Stats:
HP:            ***
Attack:       ****
Defense:     ***
Sp.Attack:  ****
Sp.Defense:*****
Speed:        *****

Capture Landorous!

You can only obtain if you have both Thundurus and Tornadus. So you either must have both Pokemon Black and White version, or you can trade globally or with a friend to obtain this Pokemon. What you have to do exactly is bring both Thundurus and Tornadus to the front of the Abundant Shrine, (located in the back end of Abundance Shrine) and Landorus will appear. Make sure you have decently leveled Pokemon, because Landorous is a high level 70.

Landorous lvl.70: Ability:Sand Force: Boosts certain moves' power in a sandstorm Moveset: Rock Slide/Earthquake/Sandstorm/Fissure
Stats:
HP:            ***
Attack:       *****
Defense:     ****
Sp.Attack:  *****
Sp.Defense:***
Speed:        ****

Capture Thundurus!/Tornadus!

As with Zekrom and Reshiram, Thundurus and Tornadus are in separate games. Thundurus is in White, and Tornadus is in Black. You meet these Pokemon in the same place in both games which is on route 7, in front of where the old ladies house is. After you encounter them there they start roaming the Unova region. (You will only see them after you defeat the elite 4 and have already captured Zekrom/Reshiram.) They have the following stats:

 Picture credit to Pokemon Company
Thundurus lvl.40(Male):Ability:Prankster:Gives priority to a status move Moveset: Revenge/Shock Wave/Heal Block/Agility
Stats:
HP:            ***
Attack:       *****
Defense:     ***
Sp.Attack:  *****
Sp.Defense:***
Speed:        *****


Tornadus lvl. 40(Male):Ability:Prankster:Gives priority to a status move Moveset: Revenge/Air Cutter/Extrasensory/Agility
Stats:
HP:            ***
Attack:       *****
Defense:     ***
Sp.Attack:  *****
Sp.Defense:***
Speed:        *****

Capture Zekrom!/Reshiram!

You can capture Zekrom in Pokemon White  version only, and Reshiram in Pokemon Black version. You can encounter these legendaries in N's castle, a place you can access after beating the elite four for the first time. These Pokemon are both lvl. 50, and have these moves and abilities:

Zekrom lvl.50: Ability:Teravolt: Moves can be used regardless of Abilities Moveset: DragonBreath/Slash/Zen Headbutt/Fusion Bolt
 Stats:
HP:            ****
Attack:       ******
Defense:     *****
Sp.Attack:  *****
Sp.Defense:****
Speed:        ****

Reshiram lvl.50: Ability:Turboblaze: Moves can be used regardless of abilities Moveset: DragonBreath/Slash/Extrasensory/Fusion Flare
Stats:
HP:            ****
Attack:       *****
Defense:     ****
Sp.Attack:  ******
Sp.Defense:*****
Speed:        ****