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I am thinking in terms of natural, in the wild type breeding. I don't think horses and donkeys do it without human intervention.
I'm not sure about that, either. But I know for a fact that horses and zebras do cross-breed without human intervention. At least occasionally. And their hybrid offspring are just as barren as mules.

The only known quantity that moves faster than
light is the office grapevine. (from Nan's fabulous Home series)