The Onager and Trimalchio's Super-Mules

There's a throwaway line in the reading for today (11 Sep), which talks briefly about Trimalchio's extravagance in terms of livestock:

"Nam mulam quidem nullam habet, quae non ex onagro nata sit."
He doesn't have a single mule that wasn't sired by an onager.

What may seem to be extravagance for extravagance's sake is actually a calculated expression of decadence.  Onagers, or wild donkeys, are very different from their domesticated brethren.  While we may think of them as slow, plodding, braying barn denizens, wild asses are skittish, and very fleet of foot-- behaving almost like antelope.  Devotees will remember Xenophon's account of donkey hunting in the desert on campaign (see Anabasis 1.5.2), and how the quarry possessed too much speed and endurance to be pursued by one horseman, forcing the Greeks to hunt one animal in a ham-handed relay.

So, Trimalchio's mules come from wild asses, not just because it's bizarre and exotic, but because that parentage would produce an offspring more prized for athletic quality.  The joke is simply that Trimalchio should have no need for a pack animal to be a fast runner (you wouldn't hitch a thoroughbred to a plow, would you?).  Like with so many other aspects of home management, Trimalchio has thrown his money around to acquire a prized tool; but not only is this super-mule unnecessary, it is wrong for the job.