Eevreon is a genetic mutation. Eevee's genetic structure is in constant flux, which is what allows it to evolve into so many different "Eeveelutions". In order for Eevee to evolve into Umbreon requires friendship evolution. However, Eevreon was about to evolve at a really young age. His age prevented a full evolution. This caused an Eevee to evolve into a pre-evolved version of Umbreon.
Eevreon often got teased by other Pokemon his age. Some said that the reason he didn't fully evolve was that he didn't have anyone who shared the friendship that he had to evolve, others said that he was a Shadow Pokemon. Then there were just some who said he was just a freak of nature. He was later found by a trainer name Alec Willey. Not knowing how to refer to this cross between an Eevee and an Umbreon, Alec just called him Eevreon.
Eevreon later took a liking to Alec's Lucario, Nightshade.
Eevreon is very friendly and slightly hyperactive. He's especially friendly with Nightshade.
Likes: Playing with his friends, having fun, running around...just about anything that burns off energy.
Dislikes: When others tease him because of being different.
As an Eevee, Eevreon had a close bond with his mother. This bond is what activated the friendship evolution. But being at such a young age, the friendship evolution didn't take full affect. So he only "semi-evolved" into a type of pre-evolved version of Umbreon.
Eevreon was often teased by other Pokemon because of his semi-evolution. Others teased him, saying things like his mother didn't love him as much as he loved her, and that's why the evolution didn't fully work, others said that he was a Shadow Pokemon. And then there were some who just called him a freak of nature.
Eventually, Eevreon was found by a trainer named Alec Willey, who cought him. Eevreon soon became friends with Alec's Lucario, Nightshade.
Mother (Vaporeon), Father (Arcanine)
The Pokemon world. (may add more info later)
Eevreon has a general Eevee body type with an Umbreon's black and yellow coloration with red eyes.