While St. Jerome is best known for his work on the Latin Vulgate translation of the Bible, he is often depicted in art with a lion.
Why is that?
The lion is not meant to be a symbolic figure, but represents a legendary account of his life.
The Golden Legend offers a brief summary of how St. Jerome became associated with a lion.
It is difficult to ascertain the truth of the story, as many miraculous stories are told about saints and their interactions with animals.
The main focus of the story is how St. Jerome calmly received the lion, not running away, willing to help an animal in need. In return for Jerome's charity, the lion became a "pet" and stayed with the monks.
The story is a reminder not to judge a book by its cover, and to receive all guests with hospitality, looking past their frightening appearances.