What is the name of the brown bull in the Táin Bó Cúailnge?

The brown bull in the Táin Bó Cúailnge is Donn Cúailnge, the Brown Bull of Cooley.

He is an exceptionally fertile stud bull whose possession drives the cattle raid at the centre of the Ulster Cycle. Queen Medb of Connacht launches the campaign because she wants to match or exceed the wealth of her husband, Ailill mac Máta, whose prized bull Finnbhennach is considered equal or superior. Donn Cúailnge is therefore both an animal and the object of a royal dispute over status and wealth.

Earlier tradition gives the bull a supernatural origin. He was once Friuch, a swineherd associated with Bodb Dearg, before being transformed into a powerful bull. During the raid, armies struggle to capture him and bring him to Connacht. Donn eventually fights Finnbhennach in a long, violent contest and kills his rival, though he is mortally wounded.

Finnbhennach is the most important possible confusion: he is the White Bull of Connacht, not the brown bull of Cooley. Bóand is a river-associated figure, while the exact spelling Donn Cúailnge reflects the name used for the bull and the Cooley region.