Leeview is getting confused between 'species': a discrete type of animal, and 'family': a group of species that are similar genetically, but often cannot interbreed.
However, closely related species of the same family may breed, and sometimes produce fertile offspring, as romany has cited above.
Given that at best, ferrets are a subspecies/variety of polecat, it'd be rather surprising if they couldn't breed and produce fertile offspring, which of course they can.
Ferrets/Polecats and mink are less closely related. So the chance of fertile offspring is less.