Ok, I will have a go at this.
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 252 Def / 252 HP
Impish Nature
- Earthquake
- Protect
- Roost
- Toxic
Our first option is Gliscor, having excellent Defense, HP and typing to deal with Lucario. It may be weak to Ice Punch, but luckily we won't be seeing any of those until the PokeBank opens in December. The set is as standard as it gets bar the max Defense to really be able to sponge Lucs STAB moves.
Landorus-Therian (M) @ Leftovers
Ability: Intimidate
EVs: 252 Atk / 252 HP / 4 SDef
Adamant Nature
- Earthquake
- Stone Edge
- U-turn
- Stealth Rock
Ok, you didn't want legendaries, but I need to mention Lando. Intimidate+superb typing allows it to both wall and dish out damage versus Lucario. Ok, you won't be able to use this until December, but it's still worth mentioning.
Golurk @ Leftovers
Ability: No Guard
EVs: 252 Atk / 252 HP / 4 SDef
Adamant Nature
- Dynamic Punch
- Earthquake
- Shadow Punch
- Stone Edge
As the last of our Lucario walls, I present Golurk. Being immune to Fighting and hitting back hard is this automatons selling-point. Sure, it's weak to Crunch, but it can still safely switch in on Close Combat and retaliate with EQ.
Dugtrio @ Focus Sash
Ability: Arena Trap
EVs: 252 Spd / 252 Atk / 4 HP
Jolly Nature
- Earthquake
- Stone Edge
- Reversal
- Sucker Punch
A more obscure counter, which prefers if it can come in unscratched. Assuming Lucario isn't carrying Extremspeed and no hazards are up, Dugtrio should have no problem dealing with Lucario.
Infernape @ Choice Scarf
Ability: Iron Fist
EVs: 252 Spd / 252 Atk / 4 SDef
Jolly Nature
- Close Combat
- Flare Blitz
- U-turn
- Thunder Punch
Hit it where it hurts, the face! Since Mega Lucario cannot have Choice Scarf, it can be quite easily outrun and revenge killed, and the sheer power of Infernape's STAB should be more than enough to deal with Lucario (as long as it's not carrying a priority move).
Moltres @ Choice Scarf
Ability: Pressure
EVs: 252 Spd / 252 SAtk / 4 Atk
Naive Nature
- Air Slash
- Flamethrower
- U-turn
- Hidden Power [Grass]
Lastly, I would like to go completely out of the normal borders. Moltres may be stuck in the pits of RU, but it does resist both of Lucarios STAB, and can easily out speed it with a Scarf and dispatch it with Flamethrower. I'm not sure if a bulky Molters would be a OHKO, THKOor a 3HKO, but for now Scarf just migth do it.
Alternatively, you could Scarf Gliscor and thusly revenge kill it (lol, I would like to see someone use that), Scarf Landorus and run max Attack, Band Golurk to assure a OHKO and Focus Sash Dugtrio and Infernape. But overall, the best way to deal with Mega Lucario would be to revenge kill it, keep hazards of your field and pray to Arceus it isn't carrying priority.
