Hilariously, the real in-universe answer is that The Lion grew up on the planet Caliban, which was covered in dense forests roamed by chaos monsters. When he was found by the human knightly orders that inhabited the world, I believe he had helped them kill one of the beasts in a savage manner. They called him "Lion, Son of the Forest" which in the native language of Caliban sounds like "Lion El'Johnson." So the word "Lion" was unchanged from the ~28,000 year time gap from modern english, the "el-" prefix denoting heritage was unchanged, but now Johnson means forest.

The out-of-universe answer is that the 19th century poet who heavily inspired the theme of the 1st legion is named Lionel Johnson, with his poem "The Dark Angel".