Evolving Eevee using the required 25 Eevee candy will turn it into one of three types at random. However, it has been discovered that you can control which type it turns into by renaming the monster beforehand:

"Sakura" to evolve into Psychic-type Espeon
"Tamao" to evolve into Dark-type Umbreon
"Rainer" to evolve into water type Vaporeon
"Sparky" to evolve into lightning type Jolteon
"Pyro" to evolve into fire type Flareon