So maybe I'm just another whiney little player complaining about everyone using the same pokemon in OU, and I need to go down to RU or something with the rest of my kind (not implying this is the only reason people play RU, either, it is quite an enjoyable, and very difficult metagame) but, in a small and probably vain effort to increase variation in the OU metagame, I would like to discuss the pokemon from the depths of OU, or from other tiers, that can now really shine in this BW2 Metagame, whether it is because of new things they can do, because of changes in the metagame around them, or a combination of the two.
So without further ado, I'll start us off with the three most obvious form the very start of BW2, and one, less obvious.
Mamoswine @ life orb
ability: thick fat
EV's:252HP/252Atk/4spe
Adamant nature
-ice shard
-icicle crash / icicle spear
-earthquake
-stone edge / superpower
A perfect counter to all of those pesky therians, as well as a sure stop to that HO team's salamence staring you down at +2atk +1 spe, mamoswine really just got it's egg moves back with thick fat, so not a ton of new stuff, but more importantly, the new metagame gave it a chance to shine, as a lot of major threats became fast, frail, and weak to ice. ice shard for the revenge kill, whichever icicle move you want, depending if you want to make sure you kill that celebi or if you want to break gliscor's subs, as well as EQ for obligatory stab moves, and then either stone edge or superpower for coverage. item can alternately be choice band to score a few more kills on things like haxorus and what not, but then it's easy for your opponent to switch in and try to use you as set up fodder.
Jolteon @ life orb
ability: volt absorb
EV's: 252spatk/4spdef/252spe
Timid nature
-volt switch
-hidden power ice
-thunderbolt
-shadow ball / signal beam
Another pokemon that saw unprecedented amounts of usage just to counter the therians, that massive speed stat, as well as that much more usable spatk stat since the metagame had begun to shift away from bulky offense, it was easily capable of revenging the therians, as well as even healing on the switch into thundurus-t's mighty thunders, this pokemon thrived on the new metagame alone, the set is unchanged from BW1. HP ice kills thund-t and land-t, volt switch for volt turning, thunderbolt kills tornadus-t and really anything you want can go in the final moveslot, I've seen anything from thunder wave to substitute. Expert belt works well if you decide to run substitute, because jolteon should really only be dishing out super effective hits anyways, as it is by no means a wall-breaker...unless that wall is skarmory or jellicent... but anyways.
Amoonguss @ black sludge
Ability: Regenerator
EV's: 252HP/28spatk/228spdef
Calm Nature
-Spore
-Giga Drain
-HP Ice
-Stun Spore / Clear Smog
Amoonguss got access to one of the games best abilities, regenerator, allowing it to recover 1/3 of its health when it switches out, and it just so happens that it's pretty good at walling many new threats, as well as some of the early countermeasures, like the above mentioned jolteon. Spore is obligatory, Giga Drain helps out with recovery, and is quite effective against many of the water types it should be switching in against. HP Ice to stop all those nasty flying types, including the therians, and then either clear smog to stop other mons from setting up on you after sleep clause activates, or stun spore to hit whatever is trying to switch in after you used spore.
Now for my personal favorite,
Toxicroak @ Black Sludge
Ability: Dry skin
EV's: 244HP/252atk/12def
Adamant nature
-bulk up
-substitute
-drain punch
-sucker punch
Toxicroak is another mon that received nothing notable from BW2 but enjoys this new metagame. Toxicroak thrives in a rain heavy metagame filled with HO teams, as it can easily begin setting up on a hazard layer, or perhaps something locked into a water move or on the dozens of mons whose movesets "hit everything but toxicroak neutrally" Get a sub, and after a bulk up or 2, it should be easy to set another sub and either start sweeping or conitinue boosting. Bulk Up and Substitute work together to keep toxicroak on the field and safe. Drain punch hits steels hard, and can help with HP recovery (as if you need more) and with all of the HO teams about, many teams will fall to something as simple as a boosted sucker puncher, and it should be used against any starmies, tornadus, or latios who want to ruin your fun.
And now... Discuss, and bring in pokemon and sets of your own!
EDIT: For whatever reason, a mod changed the title of this thread to underdogs, from dark horses, and while I believe this title is not inaccurate, and they are very similar, I would like to put the phrase "dark horses" out there, because they don't share the exact same meaning, dark horses referring to anything making a bigger splash than expected, and not just underrated pokemon, for example, noone any longer doubts mamoswine's usefulness, but before BW2, it wasn't expected to be a significant part of the metagame, so while it isn't an underdog, it is a sort of black horse. Also, this helps differentiate it from the "Underrated movesets" thread.
So without further ado, I'll start us off with the three most obvious form the very start of BW2, and one, less obvious.

Mamoswine @ life orb
ability: thick fat
EV's:252HP/252Atk/4spe
Adamant nature
-ice shard
-icicle crash / icicle spear
-earthquake
-stone edge / superpower
A perfect counter to all of those pesky therians, as well as a sure stop to that HO team's salamence staring you down at +2atk +1 spe, mamoswine really just got it's egg moves back with thick fat, so not a ton of new stuff, but more importantly, the new metagame gave it a chance to shine, as a lot of major threats became fast, frail, and weak to ice. ice shard for the revenge kill, whichever icicle move you want, depending if you want to make sure you kill that celebi or if you want to break gliscor's subs, as well as EQ for obligatory stab moves, and then either stone edge or superpower for coverage. item can alternately be choice band to score a few more kills on things like haxorus and what not, but then it's easy for your opponent to switch in and try to use you as set up fodder.

Jolteon @ life orb
ability: volt absorb
EV's: 252spatk/4spdef/252spe
Timid nature
-volt switch
-hidden power ice
-thunderbolt
-shadow ball / signal beam
Another pokemon that saw unprecedented amounts of usage just to counter the therians, that massive speed stat, as well as that much more usable spatk stat since the metagame had begun to shift away from bulky offense, it was easily capable of revenging the therians, as well as even healing on the switch into thundurus-t's mighty thunders, this pokemon thrived on the new metagame alone, the set is unchanged from BW1. HP ice kills thund-t and land-t, volt switch for volt turning, thunderbolt kills tornadus-t and really anything you want can go in the final moveslot, I've seen anything from thunder wave to substitute. Expert belt works well if you decide to run substitute, because jolteon should really only be dishing out super effective hits anyways, as it is by no means a wall-breaker...unless that wall is skarmory or jellicent... but anyways.

Amoonguss @ black sludge
Ability: Regenerator
EV's: 252HP/28spatk/228spdef
Calm Nature
-Spore
-Giga Drain
-HP Ice
-Stun Spore / Clear Smog
Amoonguss got access to one of the games best abilities, regenerator, allowing it to recover 1/3 of its health when it switches out, and it just so happens that it's pretty good at walling many new threats, as well as some of the early countermeasures, like the above mentioned jolteon. Spore is obligatory, Giga Drain helps out with recovery, and is quite effective against many of the water types it should be switching in against. HP Ice to stop all those nasty flying types, including the therians, and then either clear smog to stop other mons from setting up on you after sleep clause activates, or stun spore to hit whatever is trying to switch in after you used spore.
Now for my personal favorite,

Toxicroak @ Black Sludge
Ability: Dry skin
EV's: 244HP/252atk/12def
Adamant nature
-bulk up
-substitute
-drain punch
-sucker punch
Toxicroak is another mon that received nothing notable from BW2 but enjoys this new metagame. Toxicroak thrives in a rain heavy metagame filled with HO teams, as it can easily begin setting up on a hazard layer, or perhaps something locked into a water move or on the dozens of mons whose movesets "hit everything but toxicroak neutrally" Get a sub, and after a bulk up or 2, it should be easy to set another sub and either start sweeping or conitinue boosting. Bulk Up and Substitute work together to keep toxicroak on the field and safe. Drain punch hits steels hard, and can help with HP recovery (as if you need more) and with all of the HO teams about, many teams will fall to something as simple as a boosted sucker puncher, and it should be used against any starmies, tornadus, or latios who want to ruin your fun.
And now... Discuss, and bring in pokemon and sets of your own!
EDIT: For whatever reason, a mod changed the title of this thread to underdogs, from dark horses, and while I believe this title is not inaccurate, and they are very similar, I would like to put the phrase "dark horses" out there, because they don't share the exact same meaning, dark horses referring to anything making a bigger splash than expected, and not just underrated pokemon, for example, noone any longer doubts mamoswine's usefulness, but before BW2, it wasn't expected to be a significant part of the metagame, so while it isn't an underdog, it is a sort of black horse. Also, this helps differentiate it from the "Underrated movesets" thread.