Mega Virizion
New Ability: Justified ->
One for All
(Whenever this Pokémon's stats change, all adjacent Pokémon receive the same stat change, unless the stat change was from Mirror Armor or another Pokémon's One for All.)
Type: Grass / Fighting
New Stats:
HP: 91
Attack: 90 -> 113 (+23)
Defense: 72 -> 129 (+57)
Sp. Atk: 90 -> 110 (+20)
Sp. Def: 129
Speed: 108
(680 BST)
New moves: Horn Leech
Description: Mega Virizion's One for All ability lets it turn the tables on stat-lowering effects such as Intimidate, Sticky Web, and its own self-debuffing moves. Most Intimidate users are physical attackers, which means that when they switch in on Mega Virizion, they also weaken themselves. In a similar vein, Mega Virizion immediately inflicts -1 Speed when it switches in while Sticky Web is up, effectively neutralizing it. By switching in on a predicted Defog from a Flying-type that it outspeeds, it can guarantee a followup Stone Edge will hit, thanks to lowering the opponent's evasion. But its most interesting utility is with self-debuff moves like Close Combat and Leaf Storm - Close Combat suddenly serves as a tremendous wallbreaking tool because it drops your opponent's Defense and Sp. Defense too, while Leaf Storm demolishes your opponent's Sp. Attack stat. The tradeoff is that Mega Virizion can't safely use tools like Swords Dance or Calm Mind anymore - they benefit your opponent too!
One for All also has a bunch of really funny niche interactions that 99% of the time will never come up but are really funny if they do. King's Shield drops Aegislash and Mega Falinks' Attack. Anything with Contrary can take advantage of it, unless you're running setup. Don't let Gapdos switch in on CC or Leaf Storm unless you want it to get a free +4 Attack. Mega Galvantula and Mega Leavanny get their own Attacks lowered two stages by hitting it with Lunge. Defiant Braviary gets to clear hazards with Defog AND get a free Swords Dance.
Close Combat with Contrary Shuckle as your partner in doubles. The possibilities are endless.
Mega Articuno
New Ability: Pressure / Snow Cloak ->
Deep Fog
(When this Pokémon falls below 50% HP, it immediately uses Haze.)
Type: Ice / Flying
New Stats:
HP: 90
Attack: 85
Defense: 100 -> 115 (+15)
Sp. Atk: 95 -> 105 (+10)
Sp. Def: 125 -> 165 (+40)
Speed: 85 -> 120 (+35)
(680 BST)
New moves: Taunt
Description: Instead of going the Magic Guard / minimizing Stealth Rock damage route, this Mega Articuno takes advantage of its horrible SR weakness to activate Deep Fog. Haze on switch-in is pretty broken, but Mega Articuno's unique activation method can allow it to pretty reliably ruin the days of certain set-up sweepers, then Roost off the damage with its greatly increased Speed stat and escape. At full HP, it can directly switch in on bulky QD Volcarona's Flamethrower, Hawlucha's Close Combat, CM Reuniclus and Clefable's Thunders, SD Garchomp's Fire Fang, NP Alakazam's Psychic, and more, immediately removing their boosts if Stealth Rock is up and surviving. Of course, this isn't the optimal situation for it; it much prefers to enter battle after a faint or a slow pivot, where it can do things like clear Mega Dragonite's Dragon Dances and force it out, deny Mega Lurantis its omniboosts and threaten it with Toxic, Freeze-Dry, or Hurricane, or scare out a Swords Danced or Rock Polished Lando-T (although it can also try to switch in directly on an Earthquake if you want to push your luck). Taunt produces some interesting mindgames - do you try to set up again while they Roost off the damage they took on switch-in, or do you attack on a Taunt and remove them?
The result of all of this is that Mega Articuno adds some interesting opportunity cost to setting Stealth Rock. It might be necessary to achieve certain KOs, but you might also be ruining your win condition if you set up only to have Mega Articuno remove your boosts simply by switching in. It's held in check by a cruddy defensive typing (as always) and a pretty severe case of 4MSS - it can't run all of Roost, Toxic, Haze, Taunt, Freeze-Dry, U-turn, and Hurricane. It also flounders against stall, although it's a nice stall tool in and of itself; Haze at below 50% doesn't matter if your opponent isn't setting up anyways.
Mega Guzzlord
New Ability: Beast Boost ->
Corrosion
Type: Dark / Dragon
New Stats:
HP: 223
Attack: 113 (+12)
Defense: 89 (+36)
Sp. Atk: 127 (+30)
Sp. Def: 71 (+18)
Speed: 47 (+4)
(670 BST)
New moves: Purify, Gunk Shot
Description: Mega Guzzlord is here to spread filth and pestilence across the land. Its Dark/Dragon typing leaves it with several unfortunate weaknesses, but it makes up for these weaknesses to an extent with Corrosion. Normally, spamming Dragon-type moves like Draco Meteor leaves you wide open to incoming Steel-types - but Corrosion provides Mega Guzzlord with an interesting way to discourage Steel-type switch-ins, by threatening them with poison, leaving them hamstrung for the rest of the game if a cleric is unavailable. The addition of Gunk Shot to its movepool helps it with its other major weakness in Fairy-types, which can prey on its relatively lower Sp. Defense and double weakness. Several of those Fairies, like Tapu Fini and Clefable, either can't be poisoned even through Corrosion or don't care about it in either case, so the ability to pressure them is key.
Though Mega Guzzlord's bulk is rather tremendous, it has another big issue - it's easy to wear down. That's where Purify comes in. Corrosion means that it can poison virtually any Pokémon, which is a huge detriment to several walls like Corviknight and Toxapex, but Purify provides an incentive for Guzzlord to occasionally remove that poison, too. When used on a team with Toxic Spikes support, Purify actually becomes somewhat reliable, but it also rewards intelligent play and good predictions on both sides - a smart player may predict an incoming Purify and switch to something without a status to deny Guzzlord the recovery, or go to something that especially wants its status removed, like a burned physical attacker. Both players must evaluate which Pokémon are worth keeping statused and which can afford to lose the status in exchange for a massive HP regain. Overall, Mega Guzzlord is a tricky Mega that rewards intelligent play and accurate risk-reward calls.
(This was previously a Dark/Poison Mega with much greater bulk - it's been updated to be less overwhelming.)