In-N-Out's popular burger style started with a customer request decades ago

In-N-Out Burger is known for its simple menu, with basic burgers, fries and shakes. Yet by 2005, the fast-food chain was listing some of its most popular off-menu orders on a "Not-So-Secret Menu" online.

Among the most famous off-menu offerings is the Animal Style burger, which the company website says is a "burger of your choice with hand-leafed lettuce, tomato, a mustard-cooked beef patty; add pickle, extra spread, with grilled onions."

In-N-Out’s website says the restaurants accommodate special orders because they’re "all about making our customers happy" — and that’s just how the Animal Style burger got its start.

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"We began serving the Animal Style burger in the early 1960s when a customer watched one of our associates make his own customized burger and asked for one made the same way," In-N-Out Burger chief operating officer Denny Warnick told Fox News Digital.

"The name was created there on the spot. Over the years, many of the variations in preparation for our basic menu items were given names, usually by the customers who frequently ordered their burgers that way. The terms were merely used to describe a way of preparing an order or a menu item exactly the way a customer wanted it."

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Warnick said some of those names stuck, and today many customers order from the "secret" menu.

"Whether a ‘secret menu’ order or not, we still cook every burger fresh-to-order, exactly how the customer wants it," he said.

In addition to Animal Style, In-N-Out includes on its off-menu list Double-Meat, with two beef patties; the 3x3, with three beef patties and three slices of American cheese; the 4x4, which is the same as the 3x3 but with an added patty and cheese slice; the Grilled Cheese, which replaces a burger patty with two slices of American cheese; and Protein Style, which is "your favorite burger wrapped in hand-leafed lettuce instead of a bun."

Founders Harry and Esther Snyder opened the first In-N-Out in a suburb of Los Angeles, Calif., in 1948. According to the company’s website, it was California’s first drive-thru hamburger stand.

By 1973, there were 13 restaurants open, all in Los Angeles County.

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Today, the company is still privately owned by the Snyder family, and there are more than 400 locations in 10 states.

The off-menu options have been an open secret for decades.

The Los Angeles Times reported in 2004 that In-N-Out customers had known about ordering their burgers Animal Style "for years."

"Relish the not-so-secret menu," the Times advised in 2005, noting that customers could find the secret menu on In-N-Out’s website and suggesting they try their burgers Animal Style and fries well done.

"Then try to find the little Bible verse references hidden in the wrappers and stamped under the cups – a fun game if you’re trying to sober up from an evening out," the Times added.

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Though Animal Style is quite well known by now, the special burger is still attracting new converts.

"Got my burger Animal Style for the first time," a Redditor wrote on a subreddit two years ago. "It honestly was a game changer and is my new order. I don’t think I will ever go back."

"From LA. My parents have been going to [In-N-Out] long before I was born, and they just learned about Animal Style a month ago," another user responded. "I couldn’t believe my frickin ears."

Another off-menu order not listed on In-N-Out’s online menu is the Flying Dutchman — two patties and two cheese slices without buns or condiments — which the company says on its history page was created by former CEO Guy Snyder before he died in 1999.