All Pokémon in Pokémon Go require a small amount of candy to light up and go through a development cycle. You can earn this candy by dating them, being interested in more versions of this Pokémon, trading with them, or doing weekly research assignments. Several Pokémon have other evolutionary requirements, such as evolving at night, using an object, or near a module. There are selected Pokémon that require you to be next to the Rainy Lure module to intervene in their evolution.
At the time of writing, this is the only Pokémon that can only be used by Rainy Lure, Sliggoo to Goodry, a Dragon-type Pokémon. You can catch these Pokémon by catching Gooma, but you have to be an extremely happy trainer, which means that there is a small chance that you will meet them in the wild.
The typical evolution of Sliggoo becoming Goodra is like in traditional games, where you have to train him to level 50 and then have to wait for him to re-level when it’s raining or foggy. Sliggoo is the only Pokémon that does this. Thanks to this method, there are no other Pokémon to proceed above. We don’t know if the Niantic developers plan to add more Pokémon to it, and that could come with a Pokémon from Sword or Shield or one of the future games.