The Elephant in the Room - this expression had its origins in 1814. Ivan Andreevich Krylov, poet and fabulist, wrote a fable entitled "The Inquisitive Man" which tells of a man who goes to a museum and notices all sorts of tiny things, but fails to notice an elephant. Dostoevsky referred to this story in his novel 'Demons.' Mark Twain is credited with the first widely disseminated conceptual reference in his 1882 story "The Stolen White Elephant".
There are related expressions - for example, skeleton in the closet. There are elephant jokes. Wikipedia says: An elephant joke is a joke, almost always an absurdriddle or conundrum and often a sequence of such, that involves an elephant. Elephant jokes were a fad in the 1960s, with many people constructing large numbers of them according to a set formula. Sometimes they involve parodies or puns.
Four examples of elephant jokes are:
Q: Why did the elephant paint its fingernails red?
A: So it could hide in a cherry tree.
Q: How can you tell that an elephant is in the bathtub with you?
A: By the smell of peanuts on its breath.
Q: How can you tell that an elephant has been in your refrigerator?
A: By the footprints in the butter.
Q: What time is it when an elephant sits on your fence?