With the new games, there are bound to be some extra Pokémon forms within it. In the game, various Pokémon have brand new forms added within a variety of circumstances.

Tornadus - Therian Forme

US Name: Tornadus Therian Forme
Jp. Name: Tornadus Sacred Beast Forme

Type: Flying
Ability: Regenerator

Height: 1.4m
Weight: 63kg

First Appearance: Pokémon Black 2 & White 2

Details:
This form of Tornadus turns Tornadus into a full bird Pokémon. It's not completely obtainable within Pokémon Black 2 & White 2, but requires capture in the side game Pokémon Dream Radar. From there, it can be sent to Black 2 & White 2. The forms are interchangeable using the Reveal Glass. When switched from the Pokémon caught with Dream Radar, it has its Hidden Ability.
It has a larger focus on Defense, Special Defense and Speed

Thundurus - Therian Forme

US Name: Thundurus Therian Forme
Jp. Name: Thundurus Sacred Beast Forme

Type: Electric/Flying
Ability: Volt Absorb

Height: 3.0m
Weight: 61kg

First Appearance: Pokémon Black 2 & White 2

Details:
This form of Thundurus turns Thundurus into a flying serpent Pokémon. It's not completely obtainable within Pokémon Black 2 & White 2, but requires capture in the side game Pokémon Dream Radar. From there, it can be sent to Black 2 & White 2. The forms are interchangeable using the Reveal Glass. When switched from the Pokémon caught with Dream Radar, it has its Hidden Ability.
It has a larger focus on Special Attack

Landorus - Therian Forme

US Name: Landorus Therian Forme
Jp. Name: Landorus Sacred Beast Forme

Type: Ground/Flying
Ability: Intimidate

Height: 1.3m
Weight: 68kg

First Appearance: Pokémon Black 2 & White 2

Details:
This form of Landorus turns Landorus into a quadraped Pokémon. It's not completely obtainable within Pokémon Black 2 & White 2, but requires capture in the side game Pokémon Dream Radar. From there, it can be sent to Black 2 & White 2. The forms are interchangeable using the Reveal Glass. When switched from the Pokémon caught with Dream Radar, it has its Hidden Ability.
It has a larger focus on Attack

Black Kyurem

US Name: Black Kyurem
Jp. Name: Black Kyurem

Type: Dragon/Ice
Ability: Teravolt

Height: 3.3m
Weight: 325kg

First Appearance: Pokémon Black 2 & White 2

Details:
This form covers the cover of Pokémon Black 2 and features in the upcoming movie: Kyurem & The Sacred Swordsman Keldeo. Like Zekrom, it has its special overdrive mode where its tail gets active, but it also gets some wires extrude from its tail. It can learn the special move Freeze Shock. The form change is done using the special item, DNA Splicers which is found in Giant Chasm after capturing Kyurem. When you use this item, you have to select Kyurem and either Reshiram or Zekrom. This will merge the Pokémon together into a far stronger Pokémon. This can be reverted at any time, using the item. Any changes to Kyurem such as Level and EVs carry over to each form of Kyurem, while Reshiram or Zekrom remain untouched. For example, if you fuse a Level 70 Kyurem and a Level 70 Reshiram, and level Kyurem up to Level 92, when you defuse them, Kyurem will remain Level 92 and Reshiram will still be Level 70. However, the moves on Kyurem will reset to the last four in its level up from the current level.

White Kyurem

US Name: White Kyurem
Jp. Name: White Kyurem

Type: Dragon/Ice
Ability: Turboblaze

Height: 3.6m
Weight: 325kg

First Appearance: Pokémon Black 2 & White 2

Details:
This form covers the cover of Pokémon White 2 and features in the upcoming movie: Kyurem & The Sacred Swordsman Keldeo. Like Reshiram, it has its special overdrive mode where its tail gets active, but it also gets some wires extrude from its tail. It can learn the special move Ice Burn. The form change is done using the special item, DNA Splicers which is found in Giant Chasm after capturing Kyurem. When you use this item, you have to select Kyurem and either Reshiram or Zekrom. This will merge the Pokémon together into a far stronger Pokémon. This can be reverted at any time, using the item. Any changes to Kyurem such as Level and EVs carry over to each form of Kyurem, while Reshiram or Zekrom remain untouched. For example, if you fuse a Level 70 Kyurem and a Level 70 Reshiram, and level Kyurem up to Level 92, when you defuse them, Kyurem will remain Level 92 and Reshiram will still be Level 70. However, the moves on Kyurem will reset to the last four in its level up from the current level.

Keldeo - Resolute Form

US Name: Keldeo Resolute Form
Jp. Name: Keldeo Resolute Form

Type: Water/Fighting

Height: 1.4m
Weight: 48.5kg

First Appearance: Pokémon Black 2 & White 2

Details:
This form of Keldeo is not a massive change from the standard Keldeo, featuring a larger horn and a longer mane. It doesn't appear to have an effect on its stats but is only in this form when Keldeo knows the move "Secret Sword", which is taught in Pledge Grove