Umbreon
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It depend on what you think, I personally think that Espeon and Umbreon is the rarest evolution, because it takes you forever to get them, you need to raise their happiness to 220 ( if you don't know what it means, it's the happiness point) at day ( Espeon) or night (Umbreon ) then level up, but things change in Pokemon Heartgold/Soulsilver, Glaceon and Leafeon are the rarest in those games, because you can't get them at all! the only way you can get them is trading it into Diamond/Pearl/Platinum, evolve it, then trade back. You have to find a water/fire/thunder stone to evolve to Vaporeon, Flareon and Jolteon, and I can't find those stones!
So it really depends.
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The trainer Skier Lexi on Route 217 has Eevee's evolution in Pokemon Platinum. The trainers on Route 212 and 214 also have Eevee.
there is no earth evolution of eevee retards there is no earth evolution of eevee retards there is no earth evolution of eevee retards there is no earth evolution of eevee retards there is no earth evolution of eevee retards there is no earth evolution of eevee retards
Eevee's Dark-type evolution form is named Umbreon. In the primary core games, Eevee evolves into Umbreon when it has full Happiness during the night.
there is no rarest Pokemon you have to train and train and including shinies there is no real rarest Pokemon at all.
The only fire Pokemon in the Sinnoh region are the Chimchar-Monferno-Infernape evolution chain or the Ponyta-Rapidash evolution chain. However, once you beat the Pokemon League you can get an Eevee and evolve it to Flareon.