Wordorigins.org

I have seen the elephant is a expression denoting world-weary experience. It is an Americanism dating to the early 19th century. The elephant is metaphorical, standing it for the exotic and strange things one sees when one has experience and has seen the world.

Many associate the phrase with the Civil War. While it was certainly in use during the war and undoubtedly crops up in letters and diaries from that period, the phrase is older. From Augustus B. Longstreet’s Georgia Scenes, in a passage written in 1935:

That’s sufficient, as Tom Haynes said when he saw the elephant.

And there is G.W. Kendall’s Narrative of the Texan Santa Fé Expedition from 1844:

There is a cant expression, “I’ve seen the elephant” in very common use in Texas.

This is an American version of the older British expression to see the lions. This phrase, meaning the same thing or, in later use, meaning to see something of celebrity or note, is a reference to lions that were kept in the Tower of London and were an early tourist attraction. Those who came to London from the country were often taken to see the lions in the Tower. From Robert Greene’s 1590 Greenes Neuer Too Late:

Francesco was no other but a meere nouice, and that so newly, that to vse the olde prouerbe, he had scarce seene the Lions.

(Sources: Oxford English Dictionary, 2nd Edition; Historical Dictionary of American Slang)