In 1963 McDonald's owner Ray Kroc decided to bend to the Catholic market. Maybe it was because the president at the time was a Roman Catholic, but whatever the case Ray devised a burger that could be served to Catholics abstaining from meat on Fridays during Lent. Called the Hula Burger, this concoction was fried pineapple slices on a bun, covered in melted cheese.
Which doesn't seem like such a big surprise. I guess Kroc was ahead of his time in searching for a meatless burger, but pineapple slices? That could be interesting as its own thing, but not as a replacement for the certified Grade D meat you find in a Mickey Donald.
At around the same time that Ray was trying out the Hula Burger, a Cincinnati McDonald's franchisee was trying his own meatless concoction: the Filet-O-Fish (I'm assuming the "O" is there to entice Irish Catholics). It seemed that Catholics were going to Frisch's Big Boy (an offshoot of the Bob's Big Boy chain) because the restaraunt offered a fish sandwich. While Ray's Hula Burger failed, the Filet-O-Fish ended up a core menu item for decades.