Not much to explain here, Slowbro shrugs off all the damage Arcanine is able to do to it. Being 3HKO'd by Life Orb Wild Charge, it's an extremely reliable pivot and can retaliate with Scald to hit Arcanine extremely hard.
Tyrantrum @ Dragonium Z
Ability: Rock Head
EVs: 4 HP / 252 Atk / 252 Spe
- Rock Polish - Head Smash
- Outrage - Earthquake
Tyrantrum is a counter to Arcanine because Arcanine cannot bypass it with its moves Flare Blitz, Wild Charge and Extreme Speed due to Tyrantrums good physical bulk
and resiatance to those moves, furthermore Arcanine always takes Life Orb recoil and the recoil taken by its moves Flare Blitz / Wild Charge. Arcanine has
to heal itself at one point with Morning Sun due to the recoils it will take during that battle.
Furthermore Tyrantrum can just use Arcanine as set-up fodder and use Rock Polish to boost its speed very high, that it is capable of outpacing almost all pokémon in the RU metagame.
252 Atk Tyrantrum Head Smash vs. 0 HP / 4 Def Arcanine: 560-660 (174.4 - 205.6%) -- guaranteed OHKO
252 Atk Tyrantrum Outrage vs. 0 HP / 4 Def Arcanine: 223-264 (69.4 - 82.2%) -- 56.3% chance to OHKO after Stealth Rock
252 Atk Tyrantrum Earthquake vs. 0 HP / 4 Def Arcanine: 248-294 (77.2 - 91.5%) -- guaranteed OHKO after Stealth Rock
252 Atk Life Orb Arcanine Flare Blitz vs. 4 HP / 0 Def Tyrantrum: 48-57 (15.6 - 18.6%) -- possible 5HKO after Stealth Rock
252 Atk Life Orb Arcanine Wild Charge vs. 4 HP / 0 Def Tyrantrum: 49-58 (16 - 18.9%) -- possible 5HKO after Stealth Rock
252 Atk Life Orb Arcanine Extreme Speed vs. 4 HP / 0 Def Tyrantrum: 44-52 (14.3 - 16.9%) -- possible 6HKO after Stealth Rock
Gastrodon stonewalls Arcanine, and can beat it down with EQ or wear it down with toxic. Toxic can also cripple switch-ins. Ice beam bops Zygarde and Noivern, while recover keeps Gastrodon healthy throughout the match. This makes Gastrodon a counter to Arcanine, as it can repeatedly switch in on Arcanine throughout the match without being worn down.
Dragalge is actually a very good defensive glue in RU right now that doesn't cede momentum. It has loads of useful resistances and its stabs are hyper-boosted by Adaptability. Arcanine struggles with this mon because it can switch in and counter it, as even LO Flare blitz can't garuntee a 4HKO, while Dragalge can easily 2hko with sludge wave and Draco. What seperates Dragalge is that it doesn't just counter Arcanine when it comes in, but that it can threaten entire teams. T-Spikes are a wonderful thing to set on teams that rely on the tiers spinners for hazard removal, and Scald means that all of the tiers steel types are at risk of being severely crippled from a burn (Regi and Zong risk their Leftovers recovery, and Metagross and Escav risk their offensive capabilities). Dragalge also can still retain its offensive capabilities after an unlucky burn from Blitz that K9's physical checks would be crippled from.