Cacnea is one of many Pokémon that you can potentially catch from the wild in Pokémon Go. It is a suitable Pokémon if you are looking to add a Grass-type to your collection. When it evolves, it becomes Cacturne, a Grass and Dark-type Pokémon. While these Pokémon aren't the best choices for PvP or PvE combat, you can catch them to complete your PokéDex. How many chances do you have of encountering a shimmering Cacnea?
Unfortunately, the bright version of Cacnea didn't make it to Pokémon Go at this time. As a result, you will not be able to meet him in the wild. Cacnea initially arrived on Pokémon Go in early 2018, meaning it has been in the mobile game for nearly four years. For all that time, the glossy version was not released. Most versions of Shiny Pokémon become available within the first or two years of a Pokémon's release.
Hopefully, Niantic will make the shiny version of this Pokémon available sooner rather than later. If you're looking to catch him during his Spotlight Hour event on November 2nd, there's no chance you'll find a brilliant version. You'd better focus on catching for the event bonus everyone gets during that hour.