In-N-Out heiress reveals how rowdy customers inspired the name of the popular 'Animal Style' burger

• In-N-Out owner Lynsi Snyder reveals new details about the fast-food chain in an upcoming book.
• The iconic Animal Style burger is one of the chain's most beloved off-menu items. 
• Snyder explains how rowdy customers inspired the name of the messy, mustard-cooked burger. 

The Animal Style burger at In-N-Out is one of the chain's most legendary secret menu items. The beef patty is grilled with mustard then served with pickles, lettuce, grilled onions, and extra spread on a toasted bun.

But its origin story has been left to urban myth.