An ox owned by Gwlwlwyd Chestnut Hair.
As one of his tasks, Culhwch had to obtain this ox and yoke it together with another of Gwlwlwyd’s oxen, the Spotted Ox. The tale, however, does not mention Culhwch completing this task.
A Welsh Triad calls it one of the Three Prominent Oxen of the Island of Britain.
Sources
Culhwch and Olwen | Late 11th century
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century