Actually, to be specific, the dish I have in mind is not the traditional Eggs Benedict, which is an open-faced breakfast sandwich consisting of an English muffin covered with ham (or Canadian bacon) and poached eggs, with Hollandaise sauce poured over them. My preferred variant, consists of Andouille sausage and a poached egg with Hollandaise sauce on an English muffin, a variant I enjoy on weekends at a cafe near us called the Magnolia Grill, which is part of my favorite bookstore, Chester County Book & Music Company.
Last weekend, I started wondering just when and how Eggs Benedict was invented. Having no handy research tools for this subject but the Internet, I quickly ended up on Wikipedia, which recounted the story that, one day in 1894, a retired Wall Street stockbroker named Lemuel Benedict ordered "buttered toast, poached eggs, crisp bacon and a hooker [i.e., an amount of liquid equal roughly to the contents of a shot glass, not a prostitute] of hollandaise." The chef added the item to the menu, with the substitution of an English muffin for the toast and ham for the bacon, and the fabled dish, Eggs Benedict, was (supposedly) born.
The Food Timeline recounts this story, and adds a second one featuring a woman, a Mrs. LeGrand Benedict, who was a regular at Delmonico's in New York City. Feeling disinterested in the standard breakfast fare, she consulted with the maitre d'hotel, arriving at the now famous egg dish. There are other stories too (Wikipedia mentions one dated to about 1912), but several themes stand out: New York, and a fit of inventiveness by a (wealthy) customer.
The Lemuel Benedict story suggests that, even at the outset, substitution of ingredients was a feature of the Eggs Benedict story. I've eaten several variants of the tried and true dish (in addition to my favorite from the Magnolia Grill), but Wikipedia contains a real plethora of Benedict options. How many of these are actually prepared and eaten and how many of them are jokes devised by the author of the Wikipedia entry I can't tell(though I believe that the last one in the list must be such), but some are truly unusual. These in particular amused me:
* Eggs Sardou substitutes artichoke bottoms and crossed anchovy fillets for the English muffin and ham, then tops the hollandaise sauce with chopped ham and a truffle slice. The dish was created at Antoine's Restaurant in New Orleans in honor of the French playwright Victorien Sardou.
* Country Benedict, sometimes known as Eggs Beauregard, replaces the English muffin, ham, and hollandaise sauce with a biscuit, sausage patties, and country gravy. The poached eggs are replaced with eggs fried to choice.
* Irish Benedict replaces the ham with corned beef hash or Irish bacon.
* Dutch Benedict replaces the ham or bacon with scrapple.*
There are also variants that replace the Hollandaise sauce with another kind of sauce, or the English muffin/toast with a non-bread product, but it seems to me that once you've substituted those items, what you have is no longer anywhere near Mr. Benedict's simple hangover cure. Still, it's an interesting list, and I commend the Wikipedia article to you for that reason, if for no other.
* Scrapple is a culinary breakfast specialty of the Pennsylvania Dutch area of Pennsylvania, near where I live. Scrapple consists of pork scraps of various kinds molded together with the addition of cornmeal and flour. It is typically pan fried like bacon and served with eggs. Scrapple tends to be offputting in appearance (uncooked, it looks kind of grayish), but is tasty when fried if one likes bacon and other pork products. In texture it's a cross between a sausage and a cornbread, and its taste is unique. But it is not, by any stretch of the imagination, a health food.