Llamrai
Arthur’s mare, which he lent to Caw of Scotland for the hunting of the boar Ysgithyrwyn. It was a powerful beast that could carry four men at one time.
Arthur rode his warhorse, Llamrei, when he saved a village from a monster, an afanc. The beast lived in the lake Llyn Barfog. When Llamrei dragged the beast out of the water, she left a hoofprint on the rock Carn March Arthur.
Source
Culhwch and Olwen | Late 11th century