Mega Evolution Headcanons
Title. Some of these have canon roots in the games, while others are just made up.
Mega power and life/infinity energy are the same thing. (Dynamax energy is not the same thing, and comes from Eternatus rather than nature.)
Pokemon naturally store up infinity energy in their bodies, which is how they achieve normal evolution. Therefore, receiving too much of it can effectively force an unstable evolution where there shouldn't be one.
This is why 'mons that aren't fully evolved can't mega evolve; if something like a Fletchling was exposed to mega power, it would just evolve into a normal Fletchinder (which might cause it a bit of temporary initial pain, but wouldn't make it go rogue).
Mega stones can be created in two ways:
A) Excess life/mega energy can irradiate evolutionary stones and turn them into mega stones, which react to certain specific Pokemon.
B) The mega power absorbed by rogue mega Pokemon will naturally condense into a species-specific stone once the energy is burned off.
Once a mega stone has been created via the above methods, it can be reproduced en masse in labs (for example, Quasartico has its own internal lab that can do this).
Key stones are exclusively formed from small pieces of meteorites that were irradiated by the Ultimate Weapon. They are extremely rare to find out in nature, but have been successfully mass-produced in labs as well.
Mega stones are drained of their energy after use and will regenerate it over time. Key stones, which just siphon energy from mega stones, can be used in succession without issue.
Because mega power is held externally via stone in controlled mega evolution, a Pokemon can reject the energy and refuse to evolve if it doesn't want to (we see this with Team Flare's Mewtwo).
Mega stones can be worn by Pokemon in a variety of ways, with bracelets and collars being the most common. Many shops offer adjustable mega stone holders for Pokemon to try on.
The opposite of rogue mega evolution is known as "controlled" mega evolution (though it's considered the default, so most people don't bother saying the "controlled" part).
Rogue mega evolution hurts because a Pokemon has to bear a massive amount of energy on its own.
In controlled mega evolution, half of the mega energy is absorbed by the Pokemon via its mega stone; however, the other half is held by its trainer's key stone. The trainer bearing half the energy means that the Pokemon can access it without hurting itself.
A strong bond is needed for mega evolution because, while controlled mega evolution doesn't cause pain, the changes to a Pokemon's body and the mental effects can be hard to bear without the love of a trainer to ground them.
While mega evolution doesn't have an effect on humans, it still takes a toll on the body. Trainers who mega evolve a lot of Pokemon in a short period of time may find themselves exhausted afterward and "crashing"/taking power naps frequently.
Mega evolution lasts about 8–10 minutes normally (may be longer for legendaries); the energy is naturally released once the Pokemon reaches its tolerance limit.
However, practicing mega evolution repeatedly can gradually increase a Pokemon's tolerance towards it and allow them to stay evolved for longer. The current non-legendary record holder is a Mega Altaria that comfortably held the transformation for 23 minutes.
Practicing mega evolution frequently can likewise increase a trainer's tolerance for it and allow them to do it more often without burnout.
Some species of Pokemon handle mega evolution better than others depending on a variety of factors, including mental and physical impact. Trainers just getting into mega evolution that don't already have Pokemon that can do it are encouraged to work with Pokemon from the easy tier and work their way up. Some examples include:
Easy: Pokemon with few mental or physical issues; can be mega evolved even if their bond is weak. Dragonite, Audino, Pinsir, etc.
Average: Pokemon whose mega evolutions might have some impact on their well-being, but not to any extreme. Recommended for trainers who have used mega evolution a few times. Absol, Alakazam, Sableye, etc.
Difficult: Pokemon whose mega evolutions can easily cause them stress or discomfort; recommended only to be handled by advanced trainers with strong bonds. Salamance, Houndoom, Tyranitar, etc.
Individuals with two mega evolutions will tend to favor one form over the other. Trainers are encouraged to try out both to see which one their Pokemon prefers. (Some Pokemon will also prefer not to mega evolve in the first place.)
There is no genetic prerequirement for mega evolution. Any fully-evolved Pokemon can mega evolve if either exposed to enough life energy or the correct stone.
Rogue mega evolutions can settle into permanent evolutions if the Pokemon is left in that state for a long time. This is not a good thing, as many mega evolutions aren't suitable for day-to-day life.
Pokemon tend to not want to eat while mega evolved on account of the adrenaline involved. However, Pokemon have more of an appetite than usual after it wears off due how much energy and calories mega evolution burns.
For similar adrenaline-related reasons, mega evolved Pokemon also don't sleep.
As indicated in Mega Feraligatr's description, mega evolved Pokemon tend to have increased body temperatures and may feel "feverish" to the touch.
Mega evolution can't be used with other energy-based powerups like terastalization, dynamax, etc. because it does nothing on account of the Pokemon already using as much energy as it can. It's like trying to pour water into an already full glass.
Everstones can be used to prevent a mega evolution or even keep a Pokemon from going rogue. However, because mega evolution is so powerful, it takes multiple everstones to achieve this as one alone can't absorb enough excess energy to make an impact.