Life Force — Two Players, Into the Beast
The co-op Gradius spin-off that flew through a living creature.
Life Force (Salamander, 1988) took the Gradius engine and added the two things it needed most: a second player and a wild new theme. You fly into the body of a giant organism, blasting through pulsing, organic stages that alternate between horizontal and vertical scrolling.
Co-op shmups were rare and glorious on the NES, and Life Force was a couch favorite, doubly so once everyone learned the Konami Code granted 30 lives, exactly what you needed to survive.
Fast, weird, and gorgeous for its time. The organic level design still looks striking decades later.