Pokemon Go has updated its naming trick for guaranteeing that you evolve Eevee into a specific evolution. Fans have confirmed that players can guarantee that their Eevee evolves into Leafeon or Glaceon by using a naming trick popularized back in 2016. By giving an Eevee the nickname "Linnea", players can evolve it into Leafeon without using a Mossy Lure. Likewise, players can nickname an Eevee "Rea" to evolve it into a Glaceon. As with past nicknaming evolutions, this trick can only be used one time.
The nicknaming trick was first discovered in 2016 when Pokemon Go first came out. While Eevee's initial evolutions were seemingly random, players could guarantee its evolution by nicknaming it Rainer (for Vaporeon), Sparky (for Jolteon), or Pyro (for Flareon). The nicknames were a reference to the Eevee Brothers, a group of trainers from the original Pokemon anime series. Pokemon Go followed up by guaranteeing that you could evolve Eevee into Espeon by nicknaming it Sakura, or Umbreon by nicknaming it Tamao, which were also references to trainers seen in the Pokemon anime.
Linnea and Rea are references to Pokemon Sun and Moon, in which players faced trainers with those names (who had a Leafeon and a Glaceon respectively) during a quest to get the Eevium Z-Crystal, a special item that unlocked a species-specific Z-Move.1comments
Once players use the nicknaming trick, they'll need to evolve Eevee while in the presence of either a Mossy Lure or a Glacial Lure in order to trigger Eevee's evolution into Leafeon or Glaceon.
With the addition of Glaceon and Leafeon to the game, Pokemon Go is only missing the Fairy-type Eevee evolution Sylveon for players to complete their Eevee collection. However, players may have to wait a bit, as Sylveon is a "Gen 6" Pokemon that won't likely be added until 2020 or later.