The animal style burger is defined by cooking the patty in mustard. Where a standard burger gets a clean sear and the condiments arrive cold and afterward, this build smears yellow mustard onto the raw patty before it hits the flat-top, so the mustard fries into the crust rather than sitting on the bun. That seared mustard layer is the whole identity: it caramelizes against the beef, turns sharp and slightly bitter at the edges, and becomes part of the meat instead of a streak applied later. Everything else in the build is an amplification of the standard frame rather than a departure from it.
The craft is in sequence and excess. The mustard has to go on before the cook, not after, because the entire point is that it browns into the crust at the same time the beef does; applied cold at the end it would be a different, milder sandwich. Around that hot decision the build deliberately overloads the cool counter that a plain cheeseburger keeps restrained: extra spread, the mayonnaise-and-relish style sauce, laid on heavier than usual, a double helping of pickles for acid, and a slice of melted cheese. The onions are griddled rather than raw, cooked down soft and sweet so they read as a layer instead of a crunch, which trades the bite of raw onion for caramelized depth. The bun carries more wet cargo than a standard build, so it leans harder on the soft, slightly sweet roll that compresses to the patty and soaks the bleed without giving way. It is assembled fast and eaten fast, because a sandwich this wet does not hold.
The variations belong to the same secret-menu logic of modifying a fixed build rather than inventing a new one. Protein style swaps the bun for a lettuce wrap and changes the structural problem entirely. A double stacks two mustard-cooked patties and forces the bun to absorb twice the load. The grilled-onion-and-extra-spread treatment migrates onto other chain burgers as a known modification. Each of those is its own build with its own logic and deserves its own article rather than being crowded in here.