Interpretation:
If your debts or fates builds up, there is a possibility that one may come back in the next lifetime as a cow or horse. Remember, it is our hearts that collects our demerits from our evil actions, and releases them as fate. If our hearts have too much demerits, our fate in our next life may be as a lower species. In the Ofudesaki, the incarnation to cows is mentioned a few times.
Heaven's Truth of the Heart, Mind, & Body 