When the White Witch came to Aslan's Camp, and announced her right to Edmund's life, by the law of Narnia, the Human-Headed Bull valiantly defended Edmund, by daring her to try take it, indicating that he intended to fight her if she tried. But the Witch simply called him a fool, as she knew that mere force could not deny her her rights.
Film AdaptionsThere were no Human-Headed Bulls (the only creatures coming close was that of the Minotaurs) in the 2005 film of The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (film).
The above quote was instead said by Peter Pevensie, who defended his brother from the Witch.
The Human-Headed Bull did make an appearance, though, in the animated feature.