The borrowed expression can be an underived generic noun, as with “lion.” The noun moves from the source class to another referent through a relevant quality.
Calling a courageous person a lion is a clear form of explicit nominal metaphor. The listener evokes the animal's bravery or formidable character, while context identifies a human target.
The animal image supplies the rhetorical force; the intended human referent receives the contextually pertinent trait, not every characteristic of the source creature.