I got a couple of Dark Horses for y'all.
The first of the two is Sablenite Starmie.
Starmie @ Sablenite
Ability: Natural Cure
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Scald
- Toxic/Ice Beam/Psychic
- Recover
- Rapid Spin
Sablenite Starmie is severely underrepped due to its B- viability rank and stones on the ranking being Pidgeotite and Blue Orb. While those two allow Starmie to be a quick and very threatening offensive sweeper, Sablenite allows it to take a more defensive role. After Sablenite boosts Starmie gets very strong defensive stats of 135 for both defense and special defense, thus allowing it to be a very bulky special or physical wall. Magic Bounce gives it a counter measure to status beyond mold breaker users and can keep hazards off the field without requiring a rapid spin. Natural Cure can be used pre mega if you want to get rid of status but most of the time you will want to get the mega off as quickly as possible in order to get the needed bulk to take hits. Scald is used for chip damage, burn chance, and as a way to thaw if you randomly get frozen. The second slot can be used for many different things depending on what you want to do with it. Toxic puts your opponent on a timer but makes them unable to be burned and if used carelessly against a Magic Bounce user you will poison yourself. Ice beam hits Zygarde for good damage and for when you don't want to have to fish for burns with scald but Psychic will do more damage against your opponent most of the time. Recover is necessary for longevity and gives Starmie a niche over mons like Giratina-o as a Primaldon check due to having recovery while Giratina does not have any outside of rest. Rapid Spin allows you to get rid of hazards on your side of the field while still keeping the hazards that you put up on your opponent's side which gives Starmie a niche over Defog users but your spin will be blocked by ghost types like Gengar.
Just be careful with how you use Starmie, since it doesn't take hits nearly as well pre-mega, it can be 2hkod by attacks it could normally switch in on if mega evolved if you switch it in without mega evolving. Also be careful around Primal Groudon, although it will be the main thing you are checking with Starmie, it can be running a special set with Thunder or Solar Beam that will 2hko you if you aren't specially defensive.
The other Dark Horse mon that I have experimented with and found success with is Deoxys-Attack.
Deoxys-Attack @ Life Orb
Ability: Pressure
EVs: 4 Atk / 252 SpA / 252 Spe
Hasty Nature
- Psycho Boost
- Ice Beam/Signal Beam/Dark Pulse
- Superpower
- Extreme Speed
Deoxys-Attack is criminally underrated to the point of not even being on the viability rankings. While most people would think Deoxys-A would be outclassed by mons like Hoopa-Unbound or Gengar, Deoxys-A actually has a very strong niche in being a very fast, strong but frail attacker that chunks holes in the opposing team. Here are some calcs to give you an idea of Deoxys-A's attack power.
252 SpA Life Orb Deoxys-A Psycho Boost vs. 252 HP / 252+ SpD Primal Groudon: 294-347 (72.7 - 85.8%)
4 Atk Life Orb Deoxys-A Superpower vs. 252 HP / 252+ Def Sablenite Blissey: 369-437 (51.6 - 61.2%)
252 SpA Life Orb Deoxys-A Psycho Boost vs. 252 HP / 4 SpD Blue Orb Skarmory: 207-243 (61.9 - 72.7%)
4 Atk Life Orb Deoxys-A Superpower vs. 252 HP / 252+ Def Latiasite Heatran: 221-260 (57.2 - 67.3%)
4 Atk Life Orb Deoxys-A Superpower vs. 252 HP / 252+ Def Blue Orb Ferrothorn: 224-265 (63.6 - 75.2%)
252 SpA Life Orb Deoxys-A Psycho Boost vs. 252 HP / 4 SpD Sablenite Hippowdon: 320-378 (76.1 - 90%)
The whole point of Deoxys-Attack is that it severely weakens the big walls of the meta thus allowing its teammates to run freely without counters. While Deoxys-A didn't kill all of those walls in one shot, it did over 50% each time, which is far more than some of the non setup attackers can say bar Victini. On certain mons like Blissey, Skarmory and Hippowdon, they may be able to recover off the hit, but it gives you a free turn to switch while they recover off the damage which allows you to keep up the pressure and possibly keep the mon from completely recovering at all. Against other offensive mons Deoxys-A outspeeds with a very handy 150 base speed and clean kills them most of the time unless they pack Speed Boost or priority. Even then, Deoxys-A might outspeed them if the mon has a low enough base speed or if they are weak enough that they die to Extreme Speed. Deoxys-Attack may not sweep the opposing team, but it is strong enough to weaken or kill enough opposing walls that a teammate may sweep. Deoxys-Attack is good enough at its job that it should at least be ranked on the viability rankings.