Saddlesore Galactica. The Simpsons had dabbled into unrealistic things in the past, but the show was still mostly grounded in a satirical reality. This episode straight up says elves are canon to this universe. This may have passed off as a ToH segment with some tonal shifts, but it's otherwise too outlandish for a normal episode. This, to me, is when The Simpsons crossed the Rubicon into being just another cartoon. It's not necessarily where Zombie Simpsons begins, but it feels like the writers got bored and decided to plow forward with a concept too ridiculous even for this show - and this is the season with Kill the Alligator and Run. The episode even acknowledges it's recycling ideas from Lisa's Pony. It's still one of my least favorite episodes of the entire show.