Eevee

Eevee is a Fairy-type Pokemon. It evolves into one of eight different Pokemon, based on several different criteria.

Biology
Eevee is a mammalian, quadruped Pokémon with primarily brown fur. The tip of its bushy tail and its large furry collar are cream-colored. It has short, slender legs with three small toes and a pink paw pad on each foot. Eevee has brown eyes, long pointed ears with dark brown interiors, and a small black nose. Eevee is rarely found in the wild and is mostly only found in cities and towns. However, Eevee is said to have an irregularly shaped genetic structure that is easily influenced by its environment. This allows it to adapt to a variety of habitats by evolving.

List of Evolutions
Eevee has the most amount of potential evolutions out of any Pokemon. Here's a list of all of the evolutions of Eevee, sometimes referred in the fandom as "Eeveelutions", that existed during the time of the sixth generation:
 * Vaporeon, a Water-type Eeveelution that Eevee evolves into through the use of a Water Stone.
 * Jolteon, an Electric-type Eeveelution that Eevee evolves into through the use of a Thunder Stone.
 * Flareon, a Fire-type Eeveelution that Eevee evolves into through the use of a Fire Stone.
 * Espeon, a Psychic-type Eeveelution that Eevee evolves into by leveling up with high happiness during the day.
 * Umbreon, a Dark-type Eeveelution that Eevee evolves into by leveling up with high happiness during the night.
 * Leafeon, a Grass-type Eeveelution that Eevee evolves into by leveling up near a Moss Rock.
 * Glaceon, an Ice-type Eeveelution that Eevee evolves into by leveling up near a Ice Rock.
 * Sylveon, a Fairy-type Eeveelution that Eevee evolves into by leveling up while having at least one Fairy-type move and a minimum of two Affection Hearts in Pokemon Amie.

Trivia

 * The fan made term for Eevee's evolutions, "Eeveelutions", appeared in official media twice. The first appearance is in the official strategy guide for Pokemon Stadium 2. The second appearance is as the name of a theme deck in the TCG.
 * In any area containing a Moss or Ice Rock, Eevee cannot evolve into Espeon or Umbreon. The evolutionary requirements for Sylveon also take priority over those of Espeon and Umbreon too.
 * Early English media referred to Eevee as "Eon", which appears in all of the Eeveelutions' names.