Similar to the main Pokemon game series, the cute little Eevee has multiple evolution types in Pokemon Go. This branching evolution setup can turn it into more powerful elemental versions of itself: specifically Water, Fire, and Electric (called Vaporeon, Flareon, and Jolteon respectively).
The evolutions are usually randomly triggered, unless you use the trick below! This is confirmed to also work with Espeon and Umbreon.
There is a secret that lets you determine your Eevee‘s fate. You can choose which form to evolve your Eevee into by naming it Sakura (Espeon), Tamao (Umbreon), Pyro (Flareon), Rainer (Vaporeon) or Sparky (Jolteon). This is an Easter egg reference to trainers and the Eevee Brothers from the original anime series. Note that you have to make sure you use a capital first letter and lower case names (so, “Pyro”, not “pyro”).
Also take note that, although in Pokemon Gold and Silver Espeon could only be obtained during the day, and Umbreon at night, this is not the case in Pokemon Go. Each evolution can be obtained regardless of the time of day.
This will only work once for each of the five names. After that, no matter what you name your Eevee, the evolution will be random.