Bulbapedia counts 93 Mega Evolutions, or 96 if you count form variations. The pokee.si list has 97, because it lists Mega Meowstic male and female as two entries.
They arrived in four waves: 30 in Pokémon X and Y, 18 in Omega Ruby and Alpha Sapphire, 26 in Pokémon Legends: Z-A and 19 in its Mega Dimension DLC (22 counting form variations). Together they cover 87 species. Every table below gives the type, the base stat total (with the gain over the normal form) and the Ability.
How Mega Evolution works
Mega Evolution needs a Key Stone for the Trainer and the matching Mega Stone held by the Pokémon. Mega Rayquaza is the one exception: it needs no stone, but it must know Dragon Ascent. A Mega keeps its HP, and its base stat total is usually 100 higher than normal.
In Generations 6 and 7, each Trainer can Mega Evolve one Pokémon once per battle, and it lasts until the battle ends or the Pokémon faints. In Legends: Z-A you fill a Mega Gauge instead. The Mega form lasts a limited time, and you can Mega Evolve more than once in a battle. The full rules are on the Mega Evolution guide, and the newer system is in the Legends: Z-A guide.
Abilities don't exist in Legends: Z-A, so Mega Abilities only matter in games that have them. Several new ones, such as Piercing Drill for Mega Excadrill, were introduced in Pokémon Champions.
Mega Evolutions from X and Y
Thirty forms for 28 species. Charizard and Mewtwo each have an X and a Y version.
Mega Evolutions from Omega Ruby and Alpha Sapphire
Eighteen forms, one for each of 18 species. Mega Diancie was the first Mega of a Mythical Pokémon.
| Mega Evolution | Type | Total (+ vs normal) | Ability |
|---|---|---|---|
| Mega Beedrill | Bug/Poison | 495 (+100) | Adaptability |
| Mega Pidgeot | Normal/Flying | 579 (+100) | No Guard |
| Mega Slowbro | Water/Psychic | 590 (+100) | Shell Armor |
| Mega Steelix | Steel/Ground | 610 (+100) | Sand Force |
| Mega Sceptile | Grass/Dragon (changes) | 630 (+100) | Lightning Rod |
| Mega Swampert | Water/Ground | 635 (+100) | Swift Swim |
| Mega Sableye | Dark/Ghost | 480 (+100) | Magic Bounce |
| Mega Sharpedo | Water/Dark | 560 (+100) | Strong Jaw |
| Mega Camerupt | Fire/Ground | 560 (+100) | Sheer Force |
| Mega Altaria | Dragon/Fairy (changes) | 590 (+100) | Pixilate |
| Mega Glalie | Ice | 580 (+100) | Refrigerate |
| Mega Salamence | Dragon/Flying | 700 (+100) | Aerilate |
| Mega Metagross | Steel/Psychic | 700 (+100) | Tough Claws |
| Mega Rayquaza | Dragon/Flying | 780 (+100) | Delta Stream |
| Mega Lopunny | Normal/Fighting (changes) | 580 (+100) | Scrappy |
| Mega Gallade | Psychic/Fighting | 618 (+100) | Inner Focus |
| Mega Audino | Normal/Fairy (changes) | 545 (+100) | Healer |
| Mega Diancie | Rock/Fairy | 700 (+100) | Magic Bounce |
Mega Evolutions from Legends: Z-A
Twenty-six forms for 26 species. "(changes)" in the Type column means the Mega has a different type from the normal Pokémon.
Mega Evolutions from the Mega Dimension DLC
Bulbapedia counts 19 Mega Evolutions for 18 species here, or 22 counting form variations (two extra Tatsugiri and the Original Color Magearna). The table has 23 rows because Mega Meowstic is split by gender. This wave added the Z Mega Evolutions: Mega Absol Z, Mega Garchomp Z and Mega Lucario Z.
Strongest Megas and the odd totals
Three Megas have a base stat total of 780: Mega Mewtwo X, Mega Mewtwo Y and Mega Rayquaza. Mega Zygarde follows on 778. Many sit on 700, including Mega Dragonite, Mega Tyranitar, Mega Salamence and Mega Metagross. For the ranking of every Pokémon, see strongest Pokémon by base stats.
Two entries don't show the usual +100, because the tables compare each Mega with the default form. Mega Floette evolves from Eternal Floette (551), so its real gain is +100. Mega Zygarde comes from Complete Zygarde (708), so it gains only 70 points.
Type changes
Nineteen Megas change type. Mega Charizard X goes from Fire/Flying to Fire/Dragon, Mega Gyarados from Water/Flying to Water/Dark and Mega Aggron from Steel/Rock to plain Steel. Mega Garchomp Z drops from Dragon/Ground to pure Dragon. The rest keep the type of the Pokémon they come from.
Legendary and Mythical Megas
Twelve entries are Legendary or Mythical Pokémon: Mega Mewtwo X, Mega Mewtwo Y, Mega Latias, Mega Latios, Mega Rayquaza, Mega Heatran, Mega Darkrai, Mega Zygarde, Mega Diancie, Mega Magearna, Mega Original Magearna and Mega Zeraora. How those groups differ is covered in Legendary vs Mythical.
Primal Reversion is not a Mega
Primal Reversion is a separate transformation, not a Mega Evolution. Only two Pokémon have it.
| Primal form | Type | Total | Ability |
|---|---|---|---|
| Primal Kyogre | Water | 770 | Primordial Sea |
| Primal Groudon | Ground/Fire | 770 | Desolate Land |
Sword and Shield had no Mega Evolution. Its big battle transformation was Gigantamax, and all 32 species that can do it are in the Gigantamax list.