Arcanine (M) @ Leftovers
Trait: Intimidate
EVs: 216 Atk / 112 SAtk / 180 Spd (270 speed)
EVs: 252 Atk / 76 SAtk / 180 Spd (270 speed, use with Silk scarf)
EVs: 192 HP / 216 Atk / 40 SAtk / 60 Spd (240 speed, survives Starmie's Surf)
IVs: 30 Def / 30 SAtk / 30 SDef / 30 Spd
Lonely Nature (+Atk, -Def)
- Extreme Speed
- Fire Blast, Overheat
- Hidden Power [Fighting]
- Howl
The most generic, "historical" Arcanine of ADV. Despite having above average physical and special attack, Howl is its only means of boosting any of these. On top of this, Arcanine lacks any decent non-Fire-Type move other than Crunch, which doesn't exactly sport the best synergy with Fire. HP Fightning coming from 341 Attack is guaranteed to kill 404/256 Tyranitar after a single Howl, whereas +1 Extreme Speed obliterates uninvested Dugtrio. However, even if Arcanine had its maximum attack stat of 350, modern 253 HP Dugtrio would still survive +1 Extreme Speed 75% of the time. A similar problem arises with offensive Starmie: While +1 Extreme Speed kills it in 2 hits, Arcanine won't ever take a Hydro Pump and needs 369 HP to live through Surf, in which case it can't reach 270 speed anymore to outrun Heracross.
350 Attack with Silk Scarf can fix the Dugtrio problem most of the time:
+1 252+ Atk Silk Scarf Arcanine Extreme Speed vs. 168 HP / 0 Def Dugtrio: 245-289 (96.8 - 114.2%) -- 81.3% chance to OHKO
Overheat might even be a better option than Fire Blast, since the latter fails to kill Metagross without massive special attack investment.
Arcanine (M) @ Leftovers
Trait: Intimidate
EVs: 252 HP / 68 SAtk / 188 Spd (300 speed)
IVs: 30 Atk / 30 SAtk
Timid Nature (+Spd, -Atk)
EVs: 252 HP / 80 SAtk / 176 Spd (270 speed)
IVs: 30 Atk / 30 SAtk
Modest Nature (+SAtk, -Atk)
EVs: 32 HP / 252 SAtk / 224 Spd (310 speed, for use in ADV UU)
IVs: 30 Atk / 30 SAtk
Timid Nature (+Spd, -Atk)
- Fire Blast
- Hidden Power [Grass], Roar, Substitute, Thief (No Item), Charm, Crunch (UU only)
- Protect
- Toxic
Since the Howl variant fizzles out against more than half of ADV OU, this might be Arcanine's best shot at doing anything at all, even if it's just spreading Toxic and being a nuisance in the very late stages of the game. However, Charizard and Moltres are still far more powerful (on top of their better typing), Ninetales has better status conditions to spread (both burn and sleep) and Camerupt has Explosion and immunities to both Thunderbolt and Sand Stream. The only advantage Arcanine has over its fellow Fire Types is its ability to better match up to Heracross due to its Trait. But unless you make good use of Intimidate, even Entei arguably does this better.