Dogs may all be one species but who's to say they aren't sub species amongst each other, which gives them their distinct look. like take a look at all rat snakes. they're all elaphe. thus they're all the same species, just different sub species which man has designated. i don't see the difference between crossing dogs or cats and crossing snakes or other reptiles.
Hybrids do occur in nature (such as ratsnakes and kings crossing), and locality species do not pay attention to borders so who knows if it's what people say it is.