I dunno if this has been brought up before... but I noticed that the obelisk of the By'laggo from Through the Ermine looks a lot like a Fetch... with longer horns perhaps...
In fact... the obelisk looks almost exactly like the "spirit" form of the Fetch that is released and cast into the void when their stolen skins are torn. By'laggo is said to be part of the breath and feeds on the last exhalation of the living.
Perhaps By'laggo is some sort of Fetch Lord... and the Bre'unor, who seem to have horns, might be some sort of Fetch-Human hybrid or Fetch-Fir'Bolg hybrids. Perhaps the Bre'Unor were once a tribe of Fir'Bolg who came under the sway of the Fetch. They may have had some part in Connacht/Balor's summoning of the Fetch.