Greylings and grey dwarves are "magical" beings made of wood which is why they are weak vs axes and terribly afraid of fire (torches work great, as weapons against them)

They come out especially often when trees are being cut down (cousins perhaps?) and at night.

They are somewhat nothing more than a slight nuisance, but also serve to help train up your combat skills, so while many people become irritated by them, I see them as an opportunity to gain some combat skills and free resources.