I've heard of Animal Style (mustard-cooked beef patty, pickles, extra spread and grilled onions) and Protein Style (lettuce-wrapped burger), but never Monkey Style.
The beauty of the West Coast burger chain's simple menu is the different tiers of customization you can accomplish with a simple base of hamburgers, cheeseburgers, fries and drinks.
In-N-Out still claims they have no "secret menu," although they've acknowledged a fair amount of their off-menu items through a page on their website. It wasn't until a few days ago that I overheard a Twitter conversation about a style of burger I'd never heard of: Monkey Style.
Apparently, getting your burger "Monkey Style" involves topping your sandwich with Animal Style fries (french fries, cheese, grilled onions and spread). Our particular quest to In-N-Out was fruitful, but we've heard that many cashiers aren't familiar with Monkey Style, or simply shy away from creating it due to its possible disruptive nature to the back of the house.
Feast your eyes on the Monkey Style burger:(See: 1000 Instances of In-N-Out #MonkeyStyle, Here are the Dirtiest, Filthiest & Most Delicious) (See: The 8 Longest In-N-Out Lines According to Google Maps)