Eevee is a cute small fox Pokemon that evolves into eight different stages through different types of stones and other means. These are the following ways Eevee can evolve:
Vaporeon when exposed to a water stone.
Jolteon when exposed to a thunder stone.
Flareon when exposed to a fire stone.
Espeon when leveled up with high friendship during the daytime, including in the morning.
Umbreon when leveled up with high friendship during the nighttime.
Leafeon when leveled up near a moss stone.
Glaceon when leveled up near an ice rock.
Sylveon when leveled up with a fairy type move and 2 Affection Hearts.
Eevee has an unstable genetic makeup that suddenly mutates due to the environment in which it lives. Radiation from various stones causes this Pokemon to evolve.

To acquire an Eevee so that you can evolve it to one of the eight stages:

Red/Blue/Yellow/FireRed/LeafGreen: Received from a person in the Celadon Condominiums
Gold/Silver/Crystal/HeartGold/SoulSilver: Received from Bill in Goldenrod City
Ruby/Saphire/Emerald/Colosseum/Black/White: Receive from a trade
XD: Starter Pokemon
Diamond/Pearl/Platinum: Received from Bebe in Hearthome City or Trophy Garden
Black 2/White 2: Received from Amanita in Castelia City
X/Y: Find in Route 10, Friend Safari(Normal), Trade any Pokemon in a hotel
Omega Ruby/Alpha Saphire: Find in Route 116 (Hidden Pokemon)

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