There is actually a substantial debate on whether wolves and dogs are the same species or separate, precisely because they produce fertile offspring so easily.

There is actually a substantial debate on whether wolves and dogs are the same species or separate, precisely because they produce fertile offspring so easily. While wolves are firmly identified as Canis lupus, you can find modern sources calling dogs both Canis familiaris (a separate species), and Canis lupus familiaris (a subspecies of wolf). Probably the better example you could give is bears, where brown and polar bears are always considered separate species, but they do successfully interbreed, even in the wild occasionally. The species concept is a little too vague, and essentially the difference between two closely related species and two distant varieties of the same species is fuzzy and often historically biased (if the two populations were considered separate species for a long time, there is large inertia against re-categorizing them as a single species when evidence of genetic similarity and interbreeding arises).