Acid Rain is weather conditions caused by using Pursuit and KO'ing the foe as it switches out while either Trick Room, Gravity, Uproar, or Hail is in effect. Acid Rain causes all weather conditions to come, it makes Pokemon take 2x damage from weather buffeting, and others. Acid rain only works under certain conditions, and the game creators may have but this in the game(s) on purpose, or it may be a glitch/easter egg.
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