This lion lived at a circus in New York City, and would participate in Shadrack the Great's stunts by allowing Shadrack to insert his head into the lion's mouth without the lion biting it off. One day, however, the lion did bite Shadrack's head off. An unknown length of time later, the lion died of unknown causes.
In both life and death, the lion looks like a perfectly normal lion.
Following its death, the lion's spirit remained on earth and unable to move on, due to unresolved issues. During Dr. Zorba's travels around the world, he met and captured the Lion Ghost and imprisoned it in his mansion, along with the ghost of Shadrack, whom it killed in October of 1898.
When Dr. Zorba was murdered and became the twelfth ghost, Cyrus Zorba and his family moved into the mansion. One day, when Buck explores the cellar and finds Shadrack's top hat and a pair of spectral viewers, the Lion Ghost and the ghost of the headless Shadrack appear, with Shadrack looking into the lion's mouth for his head.
The Lion Ghost and the other eleven ghosts presumably crossed over when Benjamin Rush died and became the thirteenth ghost.