There are several versions of the same Pokémon in Pokémon Go. For example, you have Pokémon, which started in the Kanto region, which you can also find with Alola or Galar forms, giving them a unique type and different set of moves. In traditional Pokémon games, you've gone to great lengths to find and catch that Pokémon, but in Pokémon Go, you have a limited amount of space. A mechanic to help players change the shape of a Pokémon is called shape change, and it was introduced when Furfrou came out, giving rise to its many unique tweaks. The shape-shift mechanic can cost a lot, but it could be a way to save space for all of your Pokémon.
When a Pokémon can change shape, you will be able to see it when you click on it in your Pokémon collection and it should be under the "boost" option. Whenever you want to use shape shift, it will cost a certain amount of that Pokémon's candy and then stardust. For Furfrou, it costs 25 Furfrou candies and 10.000 Stardust. For many, this may be too much.
However, the only way to acquire many of Furfrou's finishes is to have a standard Furfrou and then modify it in a specific region. For example, the standard form of Furfrou is available worldwide, but in Europe it is possible to change it to its Diamond form and, while in Japan, it can become its Kabuki form. These are only available in those specific regions, so you'll need to travel there or physically trade with another manager to unlock them.
We don't know if this will be standard practice for other Pokémon that have unique forms in the future. Developer Niantic hasn't detailed how it will work in the game, but as this mechanic becomes more common, we'll be able to provide more examples and learn more.