It's a TRUE story!!!
According to TCM.com, the story of Fagan is true. A PFC Floyd C. Humeston owned Fagan, was drafted, and brought the lion with him to base. Life Magazine covered the story in the February 12, 1951 issue.
Fagan played HIMSELF in the movie. What an amazing lion, so gentle, and so able to keep to task amidst the hubbub of a film set.
From the little TCM has, Fagan ended up living at the Monterey County Humane Society after his brief fling with Hollywood. No details as to why and what happened to Humeston.
Too bad Hollywood didn't make better use of this unusual young man and his pet lion.