In one of my favorite Christmas carols, "Good Christian Men, Rejoice," there is a line that sings, "ox and ass before him bow, and he is in the manger now." As a kid (okay, for much of my adult life, too) that line frequently made me giggle.
The truth is, though, that no nativity scene would seem complete without an ox, cow, or bull, and a donkey (or ass) present. And for good reason, as this interesting post makes clear: our Christmas hymnody is rich with references (beyond the one already mentioned, also "What Child Is This?" and older versions of "The Little Drummer Boy") and scripture itself has a compelling message about how Christ came to overcome our sin and rebellion (and the animals present at his birth signify that).
Incidentally, the referenced post also has some beautiful images of older frescos and paintings that picture ox and ass in the nativity, demonstrating that this isn't a new convention in nativity scenes (as so many of the conventions are-- witness the dozens of figures offered by Fontanini and others).
As we prepare for Christ's coming, the stark realities of the manger scene-- its humility, the absence of worldly recognition, the presence of Christ amid the dirtiness and mess of a broken world-- are a warm reminder of how much we NEED Advent.
Read: Ox and Ass at Christ’s Manger