The ox and the mule

The mule is a hybrid of horse and she-ass or donkey and mare.  Pliny says that the animals that were born from two different species are a new species and don't procreate.  The ox, a castrated bull, is a domestic animal linked to the tasks of the country and work.  It represents a heavy force, slow and obstinate, but effective. 

There is a Christian tradition in the 4th century that represents the ox and the mule, often of half a body, alongside Joseph and Mary in the birth of Jesus, as in the Altar Frontal of Avià. On the one hand, the mule symbolises the capacity of Mary to give birth being a virgin, and on the other hand, the ox, docile and meek, would represent Joseph, pious and submissive, who, without intervening in the conception of Christ, accepted God's plans. 

Furthermore, as the mule is an animal opposed to the ox, it would represent the evil aspects in front of the beneficial ones, the forces of evil vanquished by the Redeemer.  In the Rooms of the museum there are examples of these animals in the birth of Jesus, mainly in the panel painting on wood, such as the altar frontal of Cardet.