Lucky Pokemon will become sparkly, take less stardust to power up (a 50% reduction) and can be tracked from the Pokedex alongside the catch and encounter statistics. In some cases Lucky Pokemon may have special new moves.
Professor Willow has made an important new discovery! According to his research, there is a chance that Pokémon, when traded, can become sparkly Lucky Pokémon! Learn more: https://t.co/O6g3ZN7pVM #LuckyPokemon pic.twitter.com/bpwLqNFnj7— Pokémon GO (@PokemonGoApp) July 23, 2018
The older the Pokemon, the higher the chance it will turn into a Lucky Pokemon during trading.
Happy Trading. Happy Hunting!