How to add a unique shiny sparkle?

[Guardianangel93]

I'd like to create a new class of Pokemon, Alpha Pokemon. They will have increased stats and display a special effect when they appear, kind of like the shiny sparkle. For example a player might come across an Alpha Gyarados, its stronger than usual and has this unique special effect.

How can I implement this into my game? Is there a way to set up an event encounter where a special effect plays at the beginning of the battle?

I only want a few Pokemon to have this effect, they will all be encountered through events and I will create new forms for these Pokemon so the vanilla Mons stay the same.