I have not seen any evolution of Legendary Pokémon yet. I'm not talking about pseudo-legendary Pokémon.
I did some more research and found that Mewtwo can Mega Evolve, but it is not a natural Legendary Pokémon, it is a man-made clone of Mew.
I have not seen any evolution of Legendary Pokémon yet. I'm not talking about pseudo-legendary Pokémon.
I did some more research and found that Mewtwo can Mega Evolve, but it is not a natural Legendary Pokémon, it is a man-made clone of Mew.
As of Gen VIII, there are only two Legendary Pokémon capable of evolving:
Both are shown doing this in the Sun and Moon anime: Cosmog while under Lillie's ownership, and Meltan while under Ash's ownership.
The following Legendary Pokémon (discounting Mewtwo) are capable of undergoing Mega Evolution and have been shown doing so in the anime:
Groudon and Kyogre are also capable of undergoing a process called Primal Reversion that is very similar to Mega Evolution but not quite the same. They are shown doing this in the Mega Evolution Specials, Hoopa and the Clash of Ages, and in Pokémon Generations.
"Legendary Pokémon" (伝説のポケモン, Densetsu no Pokémon) is a term of art.[...] In most cases, Legendary Pokémon are exceptionally powerful, difficult to catch, and unable to evolve, and no known Legendary Pokémon is capable of breeding.
source: Wiki
The few exceptions are "Nebby" in the 7th generation evolving to Solgaleo or Lunala, depend on the game version and there's also Primal Evolution of Kyogre and Groudon in OmegaRuby and AlphaSapphire which is a different term of Mega Evolution.
The reason for the Legendary Pokemon not evolving is pretty simple since evolution is a phenomenon to make Pokemon stronger than it was and as quoted from Wikipedia the legendary Pokemon in their normal state are powerful enough and not need to be evolved to get more strength.