Arcanine @ Rocky Helmet  
Ability: Intimidate  
EVs: 252 HP / 240 Def / 16 Spe  
Impish Nature  
- Will-O-Wisp  
- Morning Sun  
- Snarl  
- Extreme Speed / Flare Blitz

Possible items: Rocky Helmet, Leftovers

Role
Arcanine can function as a physical wall. Being able to cripple the strongest physical attackers in the tier with will-o-wisp and intimidate. 

Set
Max hp and 240 def with an impish nature makes Arcanine bulky on the physical side. Being able to live the strongest physical attacks after intimidate (see calcs). 
Will-o-wisp to cripple phys attackers, Morning sun for recovery. Snarl to lower the opponent's special attack. Last slot is for either extremespeed for priority or flare blitz for stab. Rocky helmet for.... what rocky helmet is for obviously (and punishing uturn spam as well) and same goes for leftovers. 16 speed EVs to outspeed neutral 0EV magearna.

Calcs (Inherited 120 Defense)
-1 252 Atk Landorus-Therian Earthquake vs. 252 HP / 252+ Def Arcanine: 152-180 (39.5 - 46.8%) -- guaranteed 3HKO
-1 252 Atk Kartana Sacred Sword vs. 252 HP / 252+ Def Arcanine: 54-64 (14 - 16.6%) -- possible 6HKO
-1 252+ Atk Smeargle Extreme Speed vs. 252 HP / 252+ Def Arcanine: 67-81 (17.4 - 21%) -- possible 5HKO (Inherited 150 attack)
-1 252 Atk Tyranitar-Mega Stone Edge vs. 252 HP / 252+ Def Arcanine: 168-198 (43.7 - 51.5%) -- 64.8% chance to 2HKO after sandstorm damage
-1 252+ Atk Groudon-Primal Precipice Blades vs. 252 HP / 252+ Def Arcanine: 236-278 (61.4 - 72.3%) -- guaranteed 2HKO (morning sun heals 75% in sunlight!)
-1 252 Atk Light Ball Pikachu Volt Tackle vs. 252 HP / 252+ Def Arcanine: 186-219 (48.4 - 57%) -- 90.2% chance to 2HKO (inherited 150 attack)
-1 252 Atk Heracross-Mega Rock Blast (5 hits) vs. 252 HP / 252+ Def Arcanine: 160-190 (41.6 - 49.4%) -- guaranteed 3HKO
-1 252+ Atk Thick Club Marowak-Alola Shadow Bone vs. 252 HP / 252+ Def Arcanine: 144-171 (37.5 - 44.5%) -- guaranteed 3HKO (Inherited 150 attack)