It is the combination of a part of Riolu(head) and Eevee(body) and it evolve into a Darkremo which is part of lucario(head) + eevee(tail) + absol(body and the neck thingy) + weavile(gems for the neck gems). It can only be obtained by donation in the Pokemon battle arena, uncatchable in psychic map(even with masterball)
Darkrema does not evolve because there is no such thing as Darkrema. Edit by ChrisUltima: Actually there is on PBA(:Pokemon Battle Arena). I'm pretty sure it evolves at Lv. 40 into Darkremo. I have a Darkrema too. Darkrema doesnt evolve, even though it sounds like Darkrai.