Which of these animals is infertile?

Actually although most are infertile there are several fertile mules producing foals around the world China, Brazil and Morocco most recently. In China they are called Dragon foals.

nature's way of telling us there shouldn't be interbreeding

Player #37282474, Surely this is an example of Natural Creation ? One day a Lonely Horse was trotting through a meadow and was delighted to meet a friendly face, looking similar to himself he felt safe, they became good pals and enjoyed each others company ..And as in Just so Stories there was a Very Happy End