The Skull Bull is the tertiary antagonist of The Book of Life.
The Skull Bull is all the bulls that all the Sánchez family bullfighters have ever killed in bullfighting merged into one giant monster skeleton bull. It was first encountered by Manolo as his challenge for coming back to life. However Manolo refuses to harm it and instead subdues it using quick thinking and plays a heartfelt apology song to the Skull Bull in which he is apologizing for all the bulls the Sanchez family bullfighters have ever killed and wronged in bullfighting.
The Skull Bull, now soothed by Manolo's song is able to rest in peace and its body ends up turning into cempasuchil marigold flower petals and ashes which disappear into a white light above.