Donn Cúailnge

In the Ulster Cycle/Táin tradition attested in medieval recensions (e.g., Lebor na hUidre and later copies), Donn Cúailnge is one of two bulls reborn after a sequence of transformations: sources relate that a man named Friuch (a pig-keeper of the Munster sidh) and his rival Rucht underwent shape-changing conflicts, were reduced to worms which were swallowed by cows, and were reborn as two bulls — Donn Cúailnge and Finnbhennach. This origin situates him within otherworldly sidh-related material and the rémscéla legend-cycle elements preserved in the manuscript tradition of the Táin.

Attested descriptions identify him as a bull, specifically called 'Donn Cúailnge' — the Brown Bull of Cooley — the epithet marking both his color ('Donn' = brown) and his association with Cúailnge (Cooley). Medieval summaries and later retellings emphasize his bull-form in action (rampaging, fighting Finnbhennach) rather than extended static morphological detail.

Sources attribute to Donn exceptional stud fertility and great economic value as a breeding bull; narratively he displays animal agency and violent behavior — rampaging when warned of an approaching army, goring a herdsman, stampeding fifty heifers through a camp, killing fifty warriors, and engaging in a prolonged combat with the rival bull Finnbhennach which he kills while receiving mortal wounds himself. He also wanders the island after being wounded, allegedly inspiring place-names, and responds to otherworld figures (the Mórrígan appears as a crow and warns him). The texts do not ascribe explicit magical powers beyond these attested traits and narrative agency.