Here's my theory, and it really is one of those theories that doesn't matter much. Anyway, as we know, Mat appears to have visited both the Aelfinn and the Eelfinn - once in Tear, and once in Rhuidean. The 'finn appear to be the "snakes and foxes" that are referred to in the normal world of the WoT. Perin has had near run-ins with the 'finn, when he was chasing Slayer in Tel'aran'rhiod and Slayer escaped into the tower. Finally, Mat fought and drove off a gholam, who was only affected by the fox medallion that Mat got from the 'finn.

Actually, that was just background. My theory is that Aginor, who created the shadowspawn (Trollocs, Fades, etc.) struck some sort of deal with the 'finn and his gholam (apparently there are only a few, and Mat's "memories" in aCoS give a few details) are actually 'finn that have been released into the "real" world.

Like I said, this isn't any earthshaking "Rand really is Shaitan" type theory, just something that muddled out of my head as I've been reading the series.