Special Evolutions

{Mega Evolution}

Mega Evolution is an additional, temporary evolution form of specific Pokémon that reached their final evolution stage. They have greatly increased Combat Power, but their HP remains the same. Certain Pokémon may also change their types after being Mega Evolved.

This form of evolution only occurs in battle. Once the battle is over, they will revert to their original form.

To initiate a Mega Evolution, each Pokemon must hold their own unique Mega Stone. The stone is named after the Pokemon. Trainers must also be wearing a bracelet known as the Mega Ring.

{Bond Evolution}

Bond Evolution is a transformation affecting Pokémon that was introduced in Pokémon the Series: XY. It is said to have some connection to the Pokémon's hidden potential.

Unlike Mega Evolution or Primal Reversion, the transformation does not require any specific items for either the Trainer or the Pokémon, though in the games it requires the ability Battle Bond. So far, the only known Pokémon to have this transformation is Greninja. In this form, the strength of Greninja's legs are superior to that of an ordinary Greninja. Additionally, it gains the ability to form large-sized Water Shurikens on its back.

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Dynamax and Gigantamax are two new mechanics introduced in Pokemon Sword and Shield. 

{Dynamax}

Dynamax is a phenomenon that occurs in specific locations or the Galar Region. Every Pokemon in Sword and Shield can Dynamax. When Dynamaxing, a Pokemon grows in size and becomes much more powerful - with increased stats.

You can only Dynamax your Pokemon once per battle. Dynamax Pokemon will return to normal after three turns. 

Only trainers who possess a Dynamax Band are able to Dynamx their Pokemon.

{Gigantamax}

Gigantamax is a rare phenomenon that can only occur to certain Pokemon. It is very similar to Dynamaxing.

Gigantamax Pokemon also grow and become more powerful.

Unlike Dynamaxing, which changes the size of the Pokemon but keeps its appearance, Gigantamaxing a Pokemon changes both its size and appearance.

This replaces a Pokemon's Dynamax function.

Gygantamaxing a Pokemon boosts its stats like Dynamax Pokemon (but we assume even more so), and Gygantamaxed Pokemon will gain access to a special G-Max Move, too.

For example, Gigantamax Drednaw's  Water-type moves will be changed to G-Max Stonesurge, which not only deals Water-type damage but leaves sharp rocks around the battlefield - like Stealth Rock.

Each G-Max move is particular to a specific species of Pokemon, so these will be wildly important when choosing a Gigantamax Pokemon to add to your team.

{All Gigantamax Pokemon}

Gigantamax Snorlax

Gigantamax Charizard

Gigantamax Pikachu

Gigantamax Eevee

Gigantamax Butterfree

Gigantamax Meowth

Gigantamax Corviknight

Gigantamax Alcremie

Gigantamax Drednaw

Gigantamax Machamp

Gigantamax Gengar

Gigantamax Toxtricity

Gigantamax Melmetal

Gigantamax Coalossal

Gigantamax Sandaconda

Gigantamax Santiskorch

Gigantamax Grimmsnarl

Gigantamax Hatterene

Gigantamax Copperajah

Gigantamax Duraludon

Gigantamax Flapple/Appletun

Gigantamax Orbeetle

Gigantamax Garbodor

Gigantamax Kingler

Gigantamax Lapras

Gigantamax Inteleon

Gigantamax Cinderace

Gigantamax Rillaboom

Gigantamax Urshifu

Gigantamax Venusaur

Gigantamax Blastoise

Only these POKEMON can Gigantamax yet.

{Dynamax vs Gigantamax vs Mega Evolution}

All Pokemon can Dynamax.

Certain Pokemon species can Mega Evolve.

Only certain individuals of certain species can Gigantamax.

Dynamax Pokemon grow in size.

Mega Evolved Pokemon change appearance.

Gigantamax Pokemon grow in size and change appearance.

A Pokemon must hold a Mega Stone in order to Mega Evolve.

Pokemon do not need to hold an item to Dynamax or Gigantamax.

Key Items a Trainer holds are needed for all three mechanics.

Dynamax and Gigantamax can only be used in certain areas in the Galar Region.

Mega Evolution is historically permitted whenever, wherever.

Mega Evolution remains for the entire battle.

Dynamax and *Gigantamax last only three turns (*unconfirmed).

Mega Evolved Pokemon don't gain special moves.

Dynamax Pokemon gain Max Moves, which are more powerful versions of the  regular moves they already know.

Gigantamax Pokemon gain ultra-powerful G-Max Moves, which are unique to that Pokemon's species, and replace a certain type of move for each species.

All three mechanics boost a Pokemon's stats, but it is unclear by how much and in what way Dynamax and Gigantamax do.