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Apocalyptic is a race horse owned by Fat Tony. It is notable for having a 666 mark on its forehead.
Krusty rode Apocalyptic in the Celebrity Harness race after the previous rider dropped out. It's odds where 1000-1. Krusty gives the horse a cocktail of vitamins, caffenine, steroids and Xanax, which angered Fat Tony and led to Krusty running from mobsters and dodging bullets at a boy's birthday party.
The horse came last, taking seven minutes to finish, and afterwards, shows the crowd a "demonstration of healthy bowels".