did you notice that in gen 7 mega evolution was quietly retconned from an emotional bond-based transformation to being more of an energy-fueled mutation and generally a cruel thing to do to a pokemon? the SM and USUM pokedex entries for mega evos are pretty much all about how much pain the pokemon is in, how it’s been mutated into a grotesque form that distresses it, how it hates being in that form, etc. and none of them are positive or mention the pokemon’s bond with the trainer
Well… I’m looking through the Pokédex entries and I think it’s a bit more ambiguous than that. There are several Pokémon for whom this seems like a fair description of the Pokédex text on their Mega Evolved forms, but they’re certainly not a majority, and there are also two Mega Evolved Pokémon who explicitly like their new forms: Mega Slowbro is said to be “pretty comfortable” ensconced inside Shellder, while Mega Pinsir supposedly never touches the ground because it’s overcome with happiness at being able to fly. There are two more that explicitly cite the importance of the Pokémon’s bond with its trainer (Mega Charizard Y and Mega Gyarados). I think that pretty well rules out any general statement about what Mega Evolution is like for all Pokémon; it affects each of them differently (which, well, makes sense). But there are also those more disturbing entries referencing things like “sharp pain and suffering” or body parts becoming “misshapen.” I think in most of these cases it’s relevant that the Pokémon involved are… well, let’s just say they’re not necessarily Pokémon you’d want at a child’s birthday party. Mega Evolution is – in my opinion – an exaggeration of everything distinctive about a Pokémon. Whatever a Pokémon already does, Mega Evolution turns it up to eleven. I don’t think they were designed with the intention that they should be proper viable organisms in their own right; they’re ridiculous overpowered battle modes that are supposed to be assumed for minutes at a time, at the very most. It sort of makes sense that they should often be quite stressful. Furthermore, if you have a Pokémon already known for viciousness or destructiveness… well, let’s see what happens, starting from the ones that aren’t particularly objectionable.
There are a couple of Pokémon whose Mega Evolved entries in the Pokédex mention that they become “irritated” or stressed from the excessive power of their new forms, like Manectric and Heracross, which seems frankly fair enough to me. Kangaskhan and Absol are said to be uneasy with Mega Evolution, but both of them for reasons that are pretty clear extensions of their existing themes. Mega Kangaskhan’s entries reference the mother’s her sadness at knowing that her child will eventually leave her, which is clearly not positive, but also makes perfect sense as an aspect of Kangaskhan’s established persona. The Let’s Go Pikachu/Eevee entry also mentions the “harmonious teamwork” of the pair, which I think is hard to view negatively. Mega Absol explicitly hates Mega Evolution, but not because it hates the form itself – Absol just doesn’t like to battle because it’s a peaceful Pokémon, and Mega Evolution is only used for battling. Then there’s Mega Lucario and Mega Metagross, who don’t suffer as a result of Mega Evolution but do become merciless and cruel while in their heightened state, while Mega Banette overflows with so much malevolent power that it can actually curse its own trainer as well as its opponents. This kind of phenomenon has a respectable pedigree from the anime storyline where Korrina learns from her grandfather how to Mega Evolve her Lucario. She quickly realises that the process invariably makes it go berserk, requiring a whole new course of training so she can control it (Mega Evolution being a little bit of a double-edged sword is not actually a new idea).
Now for the unambiguously nasty ones. Again, I think most of these can actually be seen as an extension of what the Pokémon itself is already like – taken to a ludicrous and perhaps detrimental extreme.
Mega Aerodactyl’s tremendous power makes it “incredibly irritated” and seems to cause it pain… but also, Sun version has a line about “some scholars” believing that this is Aerodactyl’s “true appearance,” while Let’s Go claims that Mega Evolution “completely restored its genes.” There’s at least a suggestion here that this is somehow what Aerodactyl is meant to be like. Mega Gyarados’ entries mention the strain on its body, as well as an alteration in its brain that causes its destructive instincts to override everything else. That… would be alarming in almost any other Pokémon, but Gyarados is already defined by its phenomenal powers of destruction (and, as previously mentioned, the entry from Let’s Go explicitly refers to the Pokémon’s bond with its trainer, which can override its destructive instincts). I think the same goes for the description of Mega Tyranitar that claims “its destructive instincts are the only thing keeping it moving”; Tyranitar’s destructive instincts are already what motivates it in its normal form. Mega Houndoom experiences internal temperatures so high that they begin to melt its claws and tips of its tail, causing it pain – again, that’s worrying, but sort of makes sense in the context of what the Pokédex has always told us about the horrifying properties of Houndoom’s fire and its supernatural ability to cause lingering pain. Mega Glalie gets a gruesome description of its jaw being broken – but Glalie is already a gruesome Pokémon that is based on an evil spirit and likes to immobilise its prey before eating them alive. Also, bizarrely, the Pokédex claims that Mega Glalie is not in pain, but rather “irritated” by the inconvenience of not being able to devour its prey. Mega Sharpedo is said to experience “sharp pain and suffering” in its numerous battle scars due to the excessive energy of Mega Evolution, which kinda fits with Sharpedo’s brutal, vicious personality. Mega Scizor suffers ill effects only if it keeps battling for a long time, but when they kick in… oh boy. “The weight of its pincers becomes too much to bear” (understandable; Heracross makes a similar complaint), and “after a long time passes, its body starts to melt.” As awful as that sounds, though, Scizor’s Pokédex entries have always mentioned how much it relies on beating its wings to regulate its temperature, and its Sun version entry actually says that without its wings “its metal body would become too hot and melt in the heat of battle” – so regular Scizor has this same problem.
The last two that have something alarming in their Pokédex entries are Mega Garchomp and Mega Salamence, and I’m separating these off because I think in these cases it’s debatable how far aggressiveness or viciousness if supposed to be a core element of the design. It’s definitely in there, but I’d suggest they aren’t quite in the same category as Gyarados, Tyranitar, or Houndoom. Mega Garchomp is said to go “mad with rage” at losing its wings, which “melt” into scythes, while Mega Salamence is said to be stressed by having its wings “becoming misshapen,” which “fuels its brutality” and causes it to rampage. There is a long association between Dragon Pokémon and rage or destructiveness, and Salamence’s pride in its wings is a really important thematic element for its whole evolutionary line, but these two Mega Evolutions (and really just these two) stick out to me as being strangely and unnecessarily unpleasant for the Pokémon.
I think for me the takeaway on this is that Mega Evolution is not something to be used lightly. It’s not a toy or something to show off with. There’s a reason not all trainers can do it, and there’s a reason Pokémon don’t just stay in those forms all the time once they learn to use the power – even for the ones that positively enjoy it, it’s tiring (maybe for the trainer as well?). At the same time, though, whatever additional stress a Pokémon might suffer in its Mega Evolved form is an extension of the same problems and pitfalls its trainer will already be used to dealing with. A trainer who’s gotten this far should, in principle, be able to guide it to controlling its power and mitigating whatever downsides are involved, at least for the few minutes it will take to stomp their enemies into the ground.