Just look at the sprites:
The Incarnate and Therian forms have different Stats. Here they are:
Tornadus: 79 HP / 115 Attack / 70 Defense / 125 Special Attack / 80 Special Defense / 111 Speed
Tornadus-Therian: 79 HP / 100 Attack / 80 Defense / 110 Special Attack / 70 Special Defense / 121 Speed
Thundurus: 79 HP / 115 Attack / 70 Defense / 125 Special Attack / 80 Special Defense / 111 Speed
Thundurus-Therian: 79 HP / 105 Attack / 70 Defense / 145 Special Attack / 80 Special Defense / 101 Speed
Landorus: 89 HP / 125 Attack / 90 Defense / 115 Special Attack / 70 Special Defense / 101 Speed
Landorus-Therian: 89 HP / 145 Attack / 80 Defense / 105 Special Attack / 70 Special Defense / 91 Speed
The new forms have different abilities than the Incarnate. The Dream World one is the same, however.
DW Ability: Defiant
Thundurus-Therian: Volt Absorb
DW Ability: Defiant
Landorus: Sand Force
DW Ability: Sheer Force
Method of obtaining:
While the Incarnate forms can be caught ingame, the Therian forms must be caught in the 3DS app Dream Radar and sent over to Black and White 2, and using the Reveal Glass on it.
Roles in the Metagame:
Tornadus: Tornadus was mainly used as a Special sweeper in the Rain. Prankster made it easy to stop Hazards and boosting. Its great offensive stats work with the sweeping. Also, some Tornadus can even run Bulk Up sets with Acrobatics to surprise people.
Thundurus: Thundurus usually uses Prankster for Priority Thunder Wave, to paralyze its opponents. It has access to Nasty Plot, and with the rain provided by Kyogre, it royally rips apart teams with its impressive Special Attack.
Landorus: Landorus is usually used with one of these two sets: Choice Scarf, or Life Orb. Choice Scarf Landorus is an excellent revenge killer, especially with Sand, and has the infamous EdgeQuake combo. It could scout via U-turn and wreck Dragons with Hidden Power [Ice]. The Life Orb and Sheer Force combination does loads of damage, and it is even more threatening with Rock Polish. Generally, an Earth Power, Focus Blast, Hidden Power [Ice] and Rock Polish set is used.
Tornadus-Therian: Tornadus-T is now scouting even more due to its ability, Regenerator, which heals every time it is switched out. Hurricane is a big threat, and Life Orbs are common on these things.
Thundurus-Therian: Same as before, but now often wielding Choice Scarf, Choice Specs, or Double Dance sets.
Landorus-Therian: Intimidate brought a whole new thing to Landorus. Now, it can sport a Bulky set with Stealth Rock, providing hazards and Physical walling. Choice Banded Landorus-T is even more common, being able to break through the sturdiest of walls.
- Increased usage of Mamoswine.
- Rain teams became more popular.
- "...the superiority of Tornadus-T caused Tornadus to decline in usage to UU." ~trachy