Shiny Pokémon are the rare variant of the original Pokémon. They have a distinct sheen around them and an unusual color for them. You can catch these unique Pokémon in Pokémon Go, but they are difficult to encounter. You have a low chance of it happening and there are always techniques to try and improve. Unfortunately, those looking for a glossy version of Slugma, this Pokémon does not have a glossy version as of this writing. Slugma doesn't have a brilliant version yet, but it should reveal itself at a later event.
The developers behind Niantic typically release this Pokémon-type variant alongside one of their weekly events. For example, during the week of weather in Season of Legends, Castform's rainy day form received a bright variant and all players have a better chance of finding it in the wild. Even then, the odds were against players, and they always will be when it comes to finding a brilliant Pokémon.
Slugma will receive a bright form in the future, though. All Pokémon in the Kanto region received theirs during the Tour: Kanto event at the end of the celebration season.
Slugma is a decent Pokémon, as is its evolved form, Magcargo. Some players have tried using it in Ultra League, but it's a bit complicated and it's a niche Pokémon, so it doesn't always take off. He also struggles in the Great League. Given its overall stats and moves, you have better Fire-type Pokémon options to fight against your opponents.