It's not immediately obvious, but the species of a couple's offspring is (for the most part) entirely random. A rabbit and dog couple could birth a cat, and another a hawk, for seemingly no reason. Even within families, it's rare to find two relatives who look alike (hence why the MC states their surprise upon meeting Nem's sister). The MC's original species is ambiguous. There is an explanation for this phenomenon, but I cannot say anything further without veering into spoilers so... =V