Mega Meowscarada:
(idk what is going on but there is supposed to be Meowscarada artwork here that keeps teleporting awa….)
Ability: Magic Guard
Typing: /
HP: 76
Atk: 140 (+30)
Def: 70
SpA: 101 (+20)
SpD: 90 (+20)
Spe: 153 (+30)
New Moves: Sword Dance, Teleport
Flavor: Mega Meowscarada is a top performing magician using slight of hand and an aroma it produces to trick its audience. It often uses magic tricks to appear and disappear suddenly.
Competitive Description: The idea of Meowscarada being the "Magician Pokemon" is somewhat lacking in its gameplay. Besides Flower Trick and a few other moves, it hardly has anything to do with Magic. I always thought Meowscarada should have gotten Magic Guard as its Hidden Ability since, you know, it's a Magician. It would also be more interesting than making it Grass type Greninja too.
So that's what this Mega is for. To basically give Meowscarada a playstyle more fitting to its theme.
Magic Guard is my choice for Megascarada. Too many Megas already get Magic Bounce, and Magic Guard allows Meowscarada to be a better pivot and serves that Magician playstyle more (also Magician would be useless on a Mega too). With Magic Guard, you can completely avoid Stealth Rock and Spikes, can't be hurt by Salt Cure, can't be hurt by Poison which additionally blocks Burns and other Status, and won't take damage from Rocky Helmet. It can't use Life Orb and abuse that, however, which would have probably made it an OU staple.
Its stat bonuses are well balanced with more of a focus on Speed and Atk. +20 in SpA is mostly to prevent Meowscarada from getting too many useful stats. Its still pretty frail physically but is now a bit more specially bulky.
At 153 Spe, you reach 445 speed, putting you above base 150s and +1 85s. However you are still pretty vulnerable to priority and most Double Speed Pokemon, as well as decently fast Scarfers too.
I also gave it 2 new moves in Sword Dance and Teleport.
Sword Dance is a great option for Meowscarada in general, probably not on Protean sets since you'll turn yourself into a Normal type, but its fast and strong. Teleport is a secondary pivot option for those that want a slow pivot, but is mostly for flavor even if it can be useful if not risky.
Mega Maushold
Ability: Repopulation*
*The user and it's ally heal for 1/8th their HP at the end of the turn while the user is on the field. The user will also heal 1/16th of its own HP when its off the field.
Typing: /
HP: 74
Atk: 85 (+10)
Def: 140 (+70)
SpA: 65
SpD: 145 (+70)
Spe: 61 (-50)
New Moves: Soft-Boiled, Return/Frustration
Flavor: Mega Evolution causes Maushold to rapidly populate an excessive amount. They have so much offspring that the dense cluster of offspring shields the parents from harm. Every time the wall of offspring is damaged, Maushold eventually replaces its offspring with newborns.
(as a side note, Tandemaus is the only Pokemon that has to evolve through level up during a battle. No rare candies. That means Tandemaus is getting their freak on during battle, making the flavor of this Mega more relevant).
Competitive Description: Megashold's idea is basically what Mega Audino should have been and even more.
One thing about these Pokemon with Low BSTs that get a Mega is that they really need something to make them compete. Like Mega Mawile gets Huge Power since otherwise it'd be trash. Mega Beedrill gets the most minmaxed stats and Adaptability since it otherwise would be the lowest BST Mega. With Maushold having a low BST as well, I had to consider that its Mega would also have low stats too and needed some way to compensate for that. I also want to make a more defensive Mega Evolution, as making Megas that are more offensive is way too common and only Mega Sableye and Mega Aggron were all that good.
With Repopulation, Megashold both supports itself and its partner in Doubles extremely well. It not only heals itself when its on the field, but it also heals itself when off the field. Imo that's how Regenerator should work, but that's besides the point. The large increase in its Defenses, Fairy typing, Soft-Boiled (closest move to repairing a wall of your own children), and the passive healing it provides makes Megashold a resilient tank. While it is not invincible, it is way harder to take down.
(pretend Poison Heal is Repopulation)
252+ Atk Supreme Overlord 5 allies fainted Kingambit Iron Head vs. 252 HP / 252+ Def Mega Maushold: 254-300 (72.1 - 85.2%) -- guaranteed 2HKO after Poison Heal
+3 88 Def Zamazenta Body Press vs. 252 HP / 252+ Def Mega Maushold: 150-177 (42.6 - 50.2%) -- guaranteed 3HKO after Poison Heal
+2 252 Atk Iron Valiant Close Combat vs. 252 HP / 252+ Def Mega Maushold: 222-262 (63 - 74.4%) -- guaranteed 2HKO after Poison Heal
252+ Atk Choice Band Protosynthesis Great Tusk Headlong Rush vs. 252 HP / 252+ Def Mega Maushold: 240-283 (68.1 - 80.3%) -- guaranteed 2HKO after Poison Heal
252 SpA Gholdengo Make It Rain vs. 252 HP / 0 SpD Mega Maushold: 288-342 (81.8 - 97.1%) -- guaranteed 2HKO after Poison Heal
So yeah it's pretty bulky.
To balance things out, Megashold has a worse off Population Bomb. It can't hold Wide Lens and no longer gets a Technician boost, in addition to being slower too. You can leverage Megashold's bulk and healing in order to set up multiple Tidy Up boosts, but you can't use Population Bomb as effectively meaning you have much weaker STAB moves to work with. At least Play Rough is now STAB.
I think another point for Megashold's place is the fact that Tidy Up removes hazards for Stall teams that otherwise would be stopped by Gholdengo. Tidy Up also allows it to pressure hazard setters since letting it Tidy Up multiple times would eventually cause it to sweep on its own. You can also answer Kingambit with Megashold as it can Encore Kingambit into Sucker Punch/Sword Dance, and can do so with its better speed (but would need Speed investment. You don't freely outspeed Kingambit).
Off the field, Megashold is being useful still as Repopulation heals it, for less, but still healing itself out of danger. That makes it so Maushold can become healthier even when out of danger and doesn't have to risk being potentially KO'd by a strong move or letting the opponent set up.
Oh and don't forget about its potential in Doubles, where it would shine the most.
Mega Perrserker
Ability: Viking Fury*
*When the user is below 1/5th their Max HP or less, this Pokemon has double Speed and deals x2 damage on all moves.
Typing: /
HP: 70
Atk: 130 (+20)
Def: 120 (+20)
SpA: 70 (+20)
SpD: 80 (+20)
Spe: 70 (+20)
New Moves: N/A
Flavor: Mega Perrserker is equipped with a new layer of menacing looking armor. This armor is unique as while Perrserker wears it, the Perrserker completely ignores the pain from its injuries and causes the wear to go into a blood lust frenzy. (yes this is a Berserk reference and to real life Vikings)
Competitive Description:
Mega Perrserker like with the other low BST Megas needs to have something stand out especially with its ability. Additionally, there aren't that many abilities that make a Pokemon stronger when their health is in the Red (or 1/5th their HP), so here is a potentially strong ability to make up for its lower BST. Its a high risk high reward ability that's some what hard to activate reliably. Once it does activate, it makes Mega Perrserker a strong wincon. Double offenses and double speed makes it really good at finishing off games. Even up against bulky resists, a Mega Perrserker is still doing heavy damage.
(pretend Huge Power is Viking Fury)
252 Atk Huge Power Mega Perrserker Close Combat vs. 248 HP / 112+ Def Slowbro-Mega: 99-117 (25.1 - 29.7%) -- guaranteed 4HKO
+2 252 Atk Huge Power Mega Perrserker Close Combat vs. 248 HP / 112+ Def Slowbro-Mega: 198-234 (50.3 - 59.5%) -- guaranteed 2HKO
+2 252 Atk Huge Power Mega Perrserker Iron Head vs. 248 HP / 68+ Def Groudon-Primal: 150-177 (37.2 - 43.9%) -- guaranteed 3HKO
and non-resists:
+2 252 Atk Huge Power Mega Perrserker Iron Head vs. 252 HP / 4 Def Pecharunt: 346-408 (91 - 107.3%) -- 43.8% chance to OHKO
This seems like really overkill damage as even some of the physically bulkiest Pokemon are being 2HKO'd from +2 resisted hits, or outright being OHKO'd, but Mega Perrserker doesn't initially get this power like Mega Mawile does. It only gets such power when are really low health. Mega Perrserker is also still vulnerable to Burns as well, so its not like there is no defensive counterplay.
On the flip side, Mega Perrserker is a strong wincon, but it also isn't unstoppable after getting Viking Fury boost. Most priority moves will stop Mega Perrserker's sweep.
252+ Atk Supreme Overlord 1 ally fainted Kingambit Sucker Punch vs. 0 HP / 0 Def Mega Perrserker: 60-72 (21.3 - 25.6%) -- guaranteed OHKO
252 Atk Samurott-Hisui Aqua Jet vs. 0 HP / 0 Def Mega Perrserker: 51-60 (18.1 - 21.3%) -- 37.5% chance to OHKO
252+ SpA Raging Bolt Thunderclap vs. 0 HP / 0 SpD Mega Perrserker: 159-187 (56.5 - 66.5%) -- guaranteed OHKO
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Mega Perrserker in Grassy Terrain: 80-95 (28.4 - 33.8%) -- guaranteed OHKO
Additionally, Mega Perrserker reaches 524 Speed at max after Viking Fury boost, which can still be outspeed by +1 Base 110s and most things with Double Speed abilities like Swift Swim
An even stat spread is the most balanced stat spread you could give Perrserker with such an ability. Its not too high in 1 stat, but isn't lacking in others. It is also still somewhat useable before Viking Fury as it still has Strong Bulk and offenses and typing too. You can pivot around with U-turn and tons of resistances, deal some damage, knock off items, etc. It won't be as strong or threatening before and not compared to other Megas, but when it manages to be at lower HP, it becomes a monster.
Also that typing is deliberate. Mega Perrserker coming in against Stealth Rock will only take 0.03% damage, which avoids letting Perrserker get to 1/5th their HP after 3 Substitutes and be at exactly 1/5th HP. It needs to use 4 Substitutes and be at 3 HP.
Also it does get Flail, but it's not STAB so while a strong option, it will only be somewhat stronger than Close Combat. It's also why I'm not giving it Reversal. That would be genuine overkill and potentially broken.
Perrserker, 
Meowscarada, 
Maushold. 
