I've been thinking its from "The Word" such as "The Word of God" and thus an aspect of it would be The Word or The Wyrd
but really I have no idea...
on a non The Forest related thing, the why are fetch so alien as to be smote by Wyrd (implying that Wyrd has power to actually DO this) but the Myrkridia aren't. If Wyrd has the power to just smite things whenever he wants to wouldnt he want to protect his 'people' and kill off the Myrkridia and the Trow and all that...?
Two mini theories for that...
1. He doesn't actually smite them... But the Fetch are just really really adverse to sunlight and it causes them to die (this does not explain why they die indoors or in caves...)
2. The Myrkridia are native to the MythWorld and just were imprisoned in the Tain and then brought back from the Tain but were originally from the MythWorld so Wyrd doesn't really care...
...or a combination of both
-General Pepper