Suppose someone is known to be cowardly and flees at the first sign of danger. Calling that person “a lion” in a transparently mocking tone differs from straightforward praise. The lion ordinarily evokes courage, but here the speaker intends ridicule.
Use the same word of someone who stood firm under danger and it conveys the ordinary metaphorical compliment. An ironic label cannot be diagnosed from the borrowed word alone. The external situation, speech setting and speaker's purpose must align.
The lesson illustrates a limit of rhetorical classification: the opposite meaning is not automatic but must be made intelligible by an indication.