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As others have said, there was a myth of a dog/fox mix, but it was never proven.
A dog and a fox cannot product offspring. It *may* be possible, but highly, highly unlikely. The reason being is that dogs (and wolves, coyotes, jackals and coyotes - any Canis species) have 78 chromosomes and fox have 38. Without the same number of chromosomes, they likely cannot successfully produce offspring.
The Asian Leopard and the domestic cat were able to produce offspring because they shared the same (38) number of chromosomes. But the offspring may be infertile.