The Nativity with Cow and Donkey

The Nativity, fresco in the lower church of St Francis in Assisi by Giotto, 1310s

In the tradition of western art, the cow (or ox) and donkey are the two animals most often present in the depiction of the Nativity. 

Lambs are sometimes present as well, but they are less uniformly so in western art, and typically are found a group. Part of the reason for this is that Christ is the Lamb of God, and thus any other lambs present  reinforce the idea that the wood of the cradle is the wood of the cross, as the Church teaches. 

The cow and donkey, by contrast, are gifts of God to Man. They are often present as a single representative of their species, standing side-by-side as in the Giotto fresco above, peering close at the Christ child from a close distance, often closer than anyone else but the Virgin herself.

It is impossible to think of the history of humanity and civilization without these two animals.

They are, significantly, the first two possessions that God allows Satan to take from Job (Job 1:14-15).  I like to think they are the ones in the Nativity symbolize the same animals returned by God.