Niantic Labs CEO John Hanke confirmed there was a way to control Eevee evolutions in Pokemon Go, confirming a long rumored Easter egg in the game. Speaking at a SDCC presentation on Sunday afternoon, Hanke confirmed that naming your Eevee a certain way before evolving it will cause the Pokemon to evolve into a Vaporeon, a Jolteon or a Flareon.
The Easter egg comes from the Pokemon anime episode "The Battling Eevee Brothers" which introduced the Eevee evolutions into the show. Each Eevee brother had a different name, themed towards one of the (then) three Eevee evolutions.
Naming an Eevee in Pokemon Go after one of the brothers will allow you to control your evolution. Naming an Eevee "Rainer" will cause it to evolve into a Vaporeon, "Pyro" will evolve into a Flareon, and "Sparky" will cause it to evolve into a Jolteon.
Hanke hinted there are additional Easter eggs hidden in Pokemon Go that nobody had found yet.
Be sure to check out all of the notes on the SDCC Pokemon Go panel here.