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He was originally one of Bodb Dearg's pig-keepers, who fell out with a fellow pig-keeper. The two fought, transforming into various animal and human forms, ultimately becoming two worms which were swallowed by two cows and reborn as two bulls, Donn Cuailnge and Finnbhennach ("White-horned"). Donn belonged to Dáire mac Fiachna, a cattle-lord of Ulster; Finnbhennach was born into the herds of queen Medb of Connacht, but considered belonging to a woman beneath him and joined the herds of her husband, Ailill.
The Mórrígan had a heifer which she took to Cooley to be bulled by Donn. The result was a bull-calf which fought Finnbhennach and narrowly lost. After seeing that, Medb was determined to see Finnbhennach fight the bull-calf's sire.
When Medb discovered that owning Finnbhennach made Ailill richer than her, she resolved to even the account by possessing Donn Cuailnge. She sent messengers to Dáire with an extremely generous offer of land and treasure, and if necessary sexual favours, if he would loan the bull to her for a year. Dáire agreed. However the messengers got drunk, and one boasted that if he hadn't agreed Medb would have taken the bull by force. When Dáire heard that he backed out of the deal.
Medb raised an army to steal Donn, and marched on Ulster. The Mórrígan, in the form of a crow, warned Donn of the coming army, and he and his herds fled to Slieve Gullion. Medb ultimately tracked him down and captured him, although she lost many soldiers under his hooves.
The army was prevented from taking Donn back to Connacht by Cúchulainn on the mount of Slieve Foy, who invoked the right of single combat, defeating a series of champions in a standoff lasting months. The Battle happened between a mountain gap, now named Mebhs Gap, on the Cooley peninsula in Co. Louth, Ireland. Eventually, after a pitched battle with the Ulster forces, Medb's armies were forced to retreat, but they managed to bring Donn Cuailnge back to Cruachan. He and Finnbhennach fought. After a long and gruelling battle Donn killed his rival. Mortally wounded himself, he wandered around Ireland inspiring placenames before returning to Cooley to die.