They're not aspects, they were created from black dragon eggs as Draenor turned into Outland. Nether drakes are the immature members of the netherwing dragonflight that are believed to be the spawn of Deathwing, but were affected by the incredible energies released by the destruction of Draenor. Living in Draenor's ruins, now known as Outland, the nether drakes are available as flying mounts to swiftly transport players around the shattered world.
http://wow.gamepedia.com/Nether_drake