A shiny Pokémon is a specific Pokémon with different coloration than what is usual for its species. These mutations often have slightly better IVs (individual values) than other non-shiny Pokémon, but the difference is nearly negligible. As is normal in Pokémon games, you have a 1/8192 chance to get a shiny Pokémon. However, there are special opportunities in Zeta/Omicron during which you can obtain specific shiny Pokémon. A few Pokémon have gotten new alternate sprites for Zeta/Omicron as the official ones are very similar to their normal coloration.