Magical horse cited in Culhwch ac Olwen. Culhwch needs both the help of the hero Gwyn ap Nudd and Du, the horse of Moro Oerfeddawg, if he is to succeed in hunting the boar Twrch Trwyth.
See Rachel Bromwich, Trioedd Ynys Prydain, rev. edn. (Cardiff, 1978), 113, 536ff.