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An onchú is an animal in 
It has been used as a heraldic device, including on the battle flag of the Irish in 1595. Williams (1989) suggests that the enfield on the crest of the O'Kellys is derived from the onchú.
Irish mythology
Irish mythology is the body of myths indigenous to the island of Ireland. It was originally Oral tradition, passed down orally in the Prehistoric Ireland, prehistoric era. In the History of Ireland (795–1169), early medieval era, myths were ...
. It is listed in the 12th-century ''Cogad Gáedel re Gallaib'' (The War of the Irish with the Foreigners) amongst the venomous beasts.
An onchú is mentioned in ''Cathréim Cellaig'' (Cellach's Victory). It terrorised the land between Loch Con and Loch Cuilin. The hero Muiredach (brother of the murdered Cellach of Killala
Cellach of Killala (fl. mid-6th century) is supposed to be an early Bishop of Killala, in Ireland.
Cellach appears among the saints of the Uí Fiachrach in ''Genealogiae Regum et Sanctorum Hiberniae'', where Walsh suggests he may have been the ...
) chased it into a lake and killed it after it had killed several of his party.
A suggested, but uncertain, etymology of its name is that ''on'' is water and cú is dog – thus water-dog.
It has been used as a heraldic device, including on the battle flag of the Irish in 1595. Williams (1989) suggests that the enfield on the crest of the O'Kellys is derived from the onchú.
References
{{Reflist Irish legendary creatures Heraldic charges