Umbreon

Umbreon, art by Ken Sugimori. Sweet Rave Parties are its middle name.

Umbreon is an official Dark-type Pokémon introduced in Generation II. It is #197 in the National PokéDex.

It evolves from Eevee when leveled up with its friendship at maximum during the nighttime. However, the ambient presence of a Moss Rock or a Snow Rock will take precedence when determining the evolutionary fate of an Eevee.

And against all laws of nature the thing is a bizarrely slow kitty-tank. What the hell, Game Freak.

For more information on Umbreon, see its Bulbapedia entry.

Evolutionary Data:
Eevee → Level up with maximum friendship at nighttime → Umbreon