Cornillo was the prize cow of Lord Goryon or Lord Gast. She was black, shone as if she had been polished, and her short, curved horns sparkled in the sunlight. Gast deemed her "the finest cow in all the land!"Taran first saw Cornillo, she was in Gast's possession; after Goryon stole her and her herd, King Smoit said he would clap Gast in irons. When Taran asked why, since Goryon had stolen Gast's cow, Smoit replied that the two lords had been stealing Cornillo from each other for so long that no one knew to whom she had originally belonged. After the King declared he would take her as a war prize, Taran bade Smoit give her to Aeddan, as his precious farmland had been destroyed by the reckless fighting of Goryon and Gast.
Cornillo gave the sweetest milk: every drop pure cream; dairy maids could scarcely churn it. She led her herd better than any herdsman; and she always gave birth to twin calves. She could pull a plow or turn a grist mill at need. Of her, Gast said (perhaps exaggerating a trifle): "Gentle as a lamb! Strong as an ox! Swift as a horse and wise as an owl!"
At such heaping praise, Cornillo would often roll her eyes or chew her cud patiently, "as though hoping not to be mistaken for anything other than a cow."