2.2.5.
For the Jews, then, the donkey was their holy animal. Tacitus confirms this in the legend

of the Jews’ exodus from Egypt, in which donkeys also play a role. Why the donkey?

The solution to the riddle is an economic one. Long before camels became the typical beast of burden in the caravans, the donkey was the animal that transported goods over long distances for trade purposes. So the donkey’s status as the Jews’ holy animal indicates its economic use for long-distance trade.