Quick note: Mega Persian is jokingly referred to as "Mega Meowth" due to sharing some similarities (I brought the coin on its head back).
Anyways, enjoy :)
Mega Mewtwo Z
Dex description: Upon mega evolving, its genes have been altered, acquiring some features from Zygarde due to the Mewtonite Z being made with some of Zygarde's DNA. Mega Mewtwo Z is the most powerful out of the other two Mewtwo mega evolutions due to having some of Zygarde's power onto it through the Mewtwonite Z.
Shiny: The green (including the visor) turns aqua green (like Zygarde's cells as a shiny), while the purple turns green (like the average Mewtwo shiny)
Mega Persian
Dex description: After mega evolving, Mega Persian's entire torso has now turned into an hourglass, having real sand inside. When this pokémon loses its patience, the sand inside its body will come out, sending the golden corks in its back flying away at intense speeds.
Shiny: The brown turns into gold, while its eyes are now green, and the golden corks (and the coin above its forehead) turn silver.
Mega Delcatty
Dex description: Mega evolving has made Mega Delcatty a more majestic pokémon, with its appearance centered around sewing but also beauty. Mega Delcatty is very gentle, but when annoyed, the needles in its neck cushion fly straight towards its foe.
Shiny: All the purple turns red, and the pink turns slightly darker.
Mega Luxray
Dex description: Mega evolving has made Luxray acquire more fur and even more static, making it have more electric power enough to overpower a Mega Raichu X and a Zapdos combined. Mega Luxray is capable of making electrical fields so powerful and so large, it can paralyze every pokémon within a 100 mile radius.
Shiny: The dark blue turns orange, while the yellow and gold turn metallic blue.
