Jackie was the second of five lions used to film that iconic roar that would preface MGM's films. He was captured live in Sudan, and was trained with affection-based reinforcement, rather than the stool-and-whip charade that was so popular at the time. As such, he became very docile, even affectionate, and famously easy to work with.
He also told Death to go f* itself on at least six separate occasions.
Seriously, it's like Unbreakable if Bruce Willis played a lion, and didn't have a nemesis, because no one would dare mess with Jackie. Here's his story: