Donn Cuailnge

In the Ulster Cycle of Irish mythology Donn Cúailnge, the Brown Bull of Cooley, was an extremely fertile stud bull over whom the Táin Bó Cúailnge (Cattle Raid of Cooley) was fought.

A ninth century rémscéla or foretale recounts how the tale came to be. In the 6th century, the poet Senchán Torpéist gathered the poets of Ireland together to see if any of them knew the story of the Táin Bó Cúailnge, but they all only knew parts of it. His son Muirgen came to the grave of Fergus mac Róich and spoke a poem, and Fergus' ghost appeared to him and related the events of the Táin as they happened.

He was originally a man named Friuch, a pig-keeper, who worked for Bodb Dearg, king of the Munster sidh. He fell out with Rucht, who was a pig-keeper for Ochall Ochne, king of the Connaught sidh. 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.

...
Wikipedia