This can have a chance to dominate the Underused metagame, no questions asked. Nothing can withstand it when playing in that atmosphere. This Pokémon is quite underrated, since it is really not seen at all anywhere. It can give you a huge dent when it comes to standard play, too. Suicune might have some troubles once Poliwrath is set up, especially if Suicune lacks Roar. Suicune can't Surf it, and Ice Beam won't do much to it. A twice Calm Minded Suicune will bring Poliwrath down with Ice Beam. Poliwrath is probably suitable in the late stages of the game.
Now this looks like an ordinary Substitute + Belly Drum Water version of Charizard, but it is in some ways better. Send this on a Water Pokémon, especially if your opponent's Suicune doesn’t have Roar. The scenario might go like this: You Substitute, they switch. You Belly Drum, they attack. You get Salac boost and you can just attack. Hidden Power Ghost hits Ghosts and Psychics super effectively, but Hidden Power Rock gets Fliers. Since Poliwrath can't get over 394 Speed (393 max with Salac), the EVs suggested give just enough to beat Jolly Dugtrio and Timid Alakazam, as well as take less than 25% from Salamence's Choice Band Rock Slide (so you can switch it in after it kills something, Substitute as it attacks for less than 1/4 or switches, and then Belly Drum). With Hidden Power Rock, you'll need to increase your Speed EVs by 4 at the cost of Defense.
In UU, barring Jolly Persian and Electrode, Adamant Poliwrath with those EVs is faster than all UU Pokémon after its Salac activates, giving you some much needed hitting power.
Return hits Poisons and, most importantly, Gligar much harder than Brick Break or your Hidden Powers, hits Fliers harder than Hidden Power Ghost, and hits Psychics harder than Hidden Power Rock. It will always OHKO Gligar, but even min HP Gligar has a chance to survive Adamant Hidden Power. You can't even touch Ghosts, obviously, but in UU, Gligar, Vileplume, and Hypno are all common, and Return is the only way to cover them all. Don't use Hidden Power Rock in UU, as there are few Fliers that don't either take neutral damage from Fighting (Fearow and company), or take neutral damage from Rock (Gligar).
A great surprise set. Put up a Substitute against Waters and such, and Focus Punch away. Hidden Power Ghost is for Ghosts while Hidden Power Rock takes care of Fliers (mostly Salamence, Gyarados and Zapdos in standard, as it won't make a difference versus Fearow and Gligar, who take more damage from Focus Punch). Hypnosis helps you to set up more easily and may shut down your main counter, while Bulk Up makes Poliwrath a physical tank. If you can set up a Substitute against slower physical Pokémon and Bulk Up a few times, they're going to be in a world of pain.
Using Ice Beam on this set isn't a bad idea. It should cripple if not kill Gligar and Salamence, and it helps against Flygon and Zapdos as well.
The Speed beats out Adamant Tyranitar, letting you Brick Break before it attacks, if it chooses to stay in. It also guarantees you will be faster than Zapdos, even 252 Timid ones, after Rock Tomb lowers its Speed. Choice Band Rock Tomb 2HKOs Zapdos about half the time after Leftovers, so if you can hit it with anything, it's going down to two Rock Tombs (Rock Tomb always lowers Speed). Ice Beam is to 2HKO Salamence, who is probably going to be their first switch in to Poliwrath. Focus Punch is easy to get off against Roar Suicune, and also good against Milotic. Sleep Talk is an option to make this thing a real Milotic counter, and it can go over anything but Brick Break.
It seriously does not have great moves. If it had Rock Slide, Poliwrath could be a little more common. Surf for that average STAB. Hydro Pump is pretty strong with STAB but is somewhat inaccurate. Counter can work off nicely due to its decent HP and Defense. Ice Beam can actually suit Poliwrath quite well. It does have some troubles with Salamence, and what can be more surprising than Ice Beaming it to oblivion.
Focus on Attack, with a bit of Speed (near max for Belly Drummers, a small amount for others), and drop the rest in HP or one of the defenses.
Checks and Counters
Claydol will just take it down, as long as it has Hidden Power Rock instead of Hidden Power Ghost. Note: Jolteon, Aerodactyl, and Crobat will still be faster than Poliwrath after the Salac boost, 1 stat point faster to be precise, if you max out Speed. As such, they will be labeled as counters. Venusaur, Donphan, Skarmory to an extent, Slowbro, Weezing, Gyarados, Flying types, Metagross, Salamence, Celebi, and Dragonite hit Poliwrath hard, in any set. As in UU, without Hydro Pump, Gligar can take any abuse it gives it and finish it off. If it's running without Substitute + Salac, faster things will just dominate it such as Fearow and Electabuzz.