There are several Pokémon in Pokémon Go that have a chance to be a shiny version when you encounter it. The real trick is not just finding them, but knowing whether a Pokémon has its glossy version activated in the game or not. When it comes to Lampent, you may be fighting him in a raid and hoping for a brilliant version.
Unfortunately, Lampent and all of its current evolutions do not have a bright version in Pokémon Go. This Pokémon's initial evolution, Litwick, was initially released on Pokémon Go in September 2019. However, it has not yet received a glossy version. It didn't get its bright version during the 2021 Halloween Mischief event; Spinarak did it.
We can probably expect the bright version of this Pokémon to appear next year, hopefully. It's all in the air. Niantic does not mention or provide concrete information for a shiny version of a Pokémon unless it announces its exact release date. They typically do this right before the event stats. Most Pokémon wait about a year before receiving their bright version, and for Litwick, Lampent, and Chandelure, we may have to wait a little longer.
Litwick does not usually appear in nature. If it does, it's a rare spawn. The best way to meet him is to fight him in a raid or fight Lampent in a three star raid. When Litwick's event comes up with the bright version of him, he will likely appear in raids.