800-pound gorilla

Etymology
Attested since at least 1971, from a riddle: "Where does an 800-pound gorilla sleep?" / "Anywhere it wants."

Noun

 * 1)  An entity that dominates its environment.
 * 2)  Something dangerous, menacing, or frightening that is obvious but not addressed.
 * 1)  Something dangerous, menacing, or frightening that is obvious but not addressed.
 * 1)  Something dangerous, menacing, or frightening that is obvious but not addressed.
 * 1)  Something dangerous, menacing, or frightening that is obvious but not addressed.
 * 1)  Something dangerous, menacing, or frightening that is obvious but not addressed.

Usage notes

 * The weight in the metaphor can be varied quantitatively.
 * Occasionally other nouns are substituted, most commonly from another idiom, but also more context-specific nouns, such as  in an article on gardening.
 * Occasionally adjectives are inserted to modify gorilla to bring the metaphor closer to the context.