How can this be?
A certain large animal lives happily and thrives here on Earth. One day, every single one of these critters is wiped out by a mysterious disease which affects only this particular animal. There are none left anywhere on earth — they are all gone. About a year or so later, they begin to reappear on Earth again. How can this be?
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The animal is the Mule. Since all Mules are born sterile, you can only get a Mule by crossing a donkey with a horse. That is how the species is able to repopulate itself.