John Hanke, CEO of Niantic Labs, developer of Pokemon GO, confirmed what many players already knew regarding Eevee Evolution.
Speaking at SDCC, Hanke confirmed that naming Eevees a certain name, directly affects the type of evolution the Pokemon goes through.
What you need to know about Eevee Evolution names in Pokémon Go
Why Sakura, Tamao, Sparky, Rainer and Pyro? The original three are the names of the Eevee brothers from the Pokémon television show, who meet with Ash and the gang in episode 40 to show off their respective Eevee evolutions, with Sakura and Tamao appearing later on in the series.