Ellén Trechend is a three-headed monster referred to in Irish mythology.
Ellén Trechend name is difficult to interpret: trechend means "three-headed", but ellén is an obscure word. One translator interprets it as a "swarm of three-headed creatures"; Whitley Stokes offered a "monstrous triple-headed bird" (Old Irish én, "bird"); while T. F. O'Rahilly identifies it with Aillén, the fire-breathing monster fought by Fionn mac Cumhaill.
His appearance is subject to debate: some pretend its a three headed dragon, others a beast, others a bird, and some say it was not one but a whole swarm of three headed things.
Ellén Trechend is mentioned in the text Cath Maige Mucrima (The Battle of Mag Mucrima) as having emerged from the cave of Cruachan (Rathcroghan, County Roscommon) and laid Ireland waste until it was killed by the Ulaid poet and hero Amergin.