Four generations, four answers to the same question
Every so often the series adds a mechanic that works only in battle and only for a moment: Mega Evolution, Z-Moves, Dynamax with Gigantamax, Terastallization. Each appeared in a different generation, each leaves along with the games it debuted in, and each answers the same question: how do you give a Trainer one decision per battle that changes something, without raising everyone's stats?
Below is what these four ideas actually do to a Pokémon, worked out from the numbers on the pages of this Pokédex. The biggest surprise is waiting at Gigantamax, and it's the exact opposite of what it looks like.
Mega Evolution: a hundred points, one form per battle
Mega Evolution is the oldest of the four and the only one that has come back. For years it looked like a closed chapter of the sixth generation; today the Pokédex lists 93 Mega forms for 90 species, with the new ones coming from Legends: Z-A and its expansion. Three species have two variants to choose from: Charizard, Mewtwo, and Raichu.
The deal is the same every time, and it's surprisingly rigid. 91 of the 93 forms get exactly a hundred base stat points. Not a hundred and a few, not "about a hundred," but exactly a hundred, the same for Mega Beedrill as for Mega Rayquaza. There are two exceptions: Mega Starmie gets 140, and Mega Zygarde is measured from Complete Forme rather than the regular one, so the comparison comes out skewed.
Then there's a second thing that's easy to forget: Mega Evolution always swaps the Ability, and always for exactly one. All 93 forms have a single, predetermined Ability, with no choice between two and no Hidden Ability. Mega Venusaur drops Chlorophyll in favor of Thick Fat, and that's not a bonus on top of the one it had but a replacement.
Where those hundred points go
A hundred points is a hundred points, but they're handed out very unevenly. Below is the average gain for each stat, calculated across all 93 forms at once.
| Stat | Average gain | What it means |
|---|---|---|
| Attack | +26.5 | The biggest share of the pool, together with Sp. Atk |
| Sp. Atk | +25.9 | Practically tied with physical Attack |
| Defense | +20.0 | A Mega almost always gets tougher |
| Sp. Def | +18.1 | Same story, slightly less |
| Speed | +9.6 | Changes in 67 of 93 forms, but rarely by much |
| HP | +1.2 | Zero: apart from a single Zygarde, it doesn't move by even one point |
One practical conclusion follows from this table. Mega Evolution isn't a way to make a slow Pokémon start outspeeding things. More than half the pool goes into offense, a third into defense, and Speed gets the scraps. Exceptions exist, and they're exactly why people pick them: Mega Beedrill jumps from 75 to 145 Speed, and that's the whole reason anyone uses it. But reaching for a Mega "so it's faster" usually ends in disappointment.
HP deserves a separate mention, because its absence can be costly. Mega Evolution happens mid-battle, after the Pokémon may already have taken hits. Its stats will jump, but the damage it has already taken stays, and since max HP doesn't change, there isn't even the kind of boost Dynamax gives.
Type changes less often than you'd think
Mega Charizard X stops being Flying and becomes a Dragon type, and Mega Gyarados swaps Flying for Dark. Those are the cases everyone remembers, and that's exactly why it's easy to assume the whole mechanic works that way. Only 17 of the 93 forms change type. The other 76 go through the battle with exactly the same set of weaknesses they started it with.
When the type does change, it usually happens by adding a second type, not swapping one: Pinsir gains Flying, Mewtwo X gains Fighting, Ampharos gains Dragon. A swap happens twice: Mega Aggron gives up Rock and becomes pure Steel, and Mega Staraptor trades Normal for Fighting. After the Ability, it's the second thing worth checking before a battle, because an opponent who was counting on a type advantage can lose it halfway through a turn.
The same deal applies to two Legendaries from the third generation, although it goes by a different name. Kyogre and Groudon in their Primal forms get an even hundred points, one new Ability, and, in Groudon's case, a second type. Mechanically, it's Mega Evolution under a different name.
Z-Moves: one move instead of one form
The seventh generation went the opposite way. Instead of changing the Pokémon, a Z-Move changes one move: once per battle, the chosen move turns into a version with many times the power, and the Pokémon stays itself. The cost is the item slot. The Z-Crystal takes it over completely, and that slot usually holds something that works for the whole battle.
It's the most fleeting of the four mechanics and the only one you won't find on any page on this site. Z-Moves don't have their own pages in the move list, because they aren't separate moves; they're a recalculation of a move the Pokémon already knows. So when a move page lists a power value, it's the power of the regular version, and that's also how the damage calculator counts it.
Dynamax and Gigantamax: nothing changes in the numbers
Here's a surprise worth knowing about before you open a Gigantamax page. The Pokédex lists 34 Gigantamax forms, and all 34 have base stats, types, and Abilities identical in every respect to the base variant. Not "almost the same" and not "the same except for one," but identical, without a single exception.
The only column that moves is size. Gigantamax Venusaur stands 24 meters tall instead of two, and the tallest of all thirty-four, Gigantamax Centiskorch, is 75 meters. Weight is set the same for all of them, because the games simply don't list it for these forms.
That doesn't mean Dynamax does nothing. It means it does it outside the Pokémon's page. For three turns, current and max HP double, regular moves turn into Max Moves with an added field effect, and Gigantamax forms get their own G-Max Move in place of one of them. All of that happens in battle and has no way of making it into a base stat table, which describes the species, not the situation.
The practical conclusion is simple: Gigantamax is a look and one move, not power. If you're choosing between a regular and a Gigantamax-capable specimen of the same species, you're not choosing between a weaker one and a stronger one.
Terastallization: the type changes, not the form
The ninth generation stripped the mechanic down to a single thing. Terastallization doesn't add points, doesn't swap the Ability, and doesn't change size. It changes the type, to one that each individual Pokémon carries with it, set in advance. A Pokémon with a Fire Tera Type is Fire and only Fire after Terastallizing, no matter what it was a moment earlier.
The consequences cut both ways, and that's the whole weight of this mechanic. The bonus for a move matching the Pokémon's type carries over to the new type, so a Pokémon that was hitting at reduced strength until now suddenly hits at full strength. At the same time, all of its previous resistances disappear. A dual-type Pokémon has one type and one set of weaknesses after Terastallizing, often completely different from the one the opponent picked their attack for.
Two moves are directly tied to this mechanic, and both are on this site: Tera Blast, which takes on the Tera Type after Terastallization and picks the better of the two attacking stats, and Tera Starstorm, exclusive to a single species.
How the four differ in practice
| Mechanic | What it changes | How long | What it costs |
|---|---|---|---|
| Mega Evolution | +100 stats, one Ability, sometimes type | Until the end of the battle | The item slot (Mega Stone), once per battle |
| Z-Move | One move, one time | One turn | The item slot (Z-Crystal), once per battle |
| Dynamax / Gigantamax | Doubled HP and Max Moves | Three turns | Once per battle, item slot stays free |
| Terastallization | Type: the whole thing, swapped for one | Until the end of the battle | Once per battle, item slot stays free |
This shows why the last two turned out to matter more for teams than they first appeared to. Megas and Z-Moves have to be paid for with an item that can't be used for anything else (the only exception is Rayquaza, which needs a specific move in its moveset instead of a stone); Dynamax and Terastallization are free and come into battle alongside whatever the Pokémon is already holding.
Why the rankings leave them out by default
If you look at the stat rankings, Mega and Gigantamax forms aren't in them by default, and after all of the above it's clear that's not an oversight. Mega Mewtwo's stat total is 780 points, against 680 for the regular one. If they were counted together, the top of every category would be taken over by forms you can't keep permanently and that only last as long as the battle does. You can turn them on with a single toggle, and then you see the scale of that difference right away.
For the same reason, the percentages on Pokémon pages are calculated against base and regional forms, and Megas and Gigantamax forms aren't part of that scale. A Mega's page shows its percentages measured with the same yardstick as everything else; otherwise every regular stat would look weaker than it really is.