13 “The onagers shall take for their thirst.” By onagers he means some great beasts. For who knows not that wild asses are called onagers? He means, therefore, some great untrained ones. For the Gentiles had no yoke of the Law: many nations lived after their own customs, ranging in proud boastfulness as in a wilderness. And so indeed did all the beasts, but the wild asses are put to signify the greater sort. They too shall drink for their thirst, for for them too the waters flow.
Thence drinks the hare, thence the wild ass: the hare little, the wild ass great; the hare timid, the wild ass fierce: either sort drinks thence, but each for his thirst....So faithfully and gently does it flow, as at once to satisfy the wild ass, and not to alarm the hare. The sound of Tully's voice rings out, Cicero is read, it is some book, it is a dialogue of his, whether his own, or Plato's, or by whatever such writer: some hear that are unlearned, weak ones of less mind; who dares to aspire to such a thing?
It is a sound of water, and that perchance turbid, but certainly flowing so violently, that a timid animal dare not draw near and drink. To whom sounds a Psalm, and he says, It is too much for me? Behold now what the Psalm sounds; certainly they are hidden mysteries, yet so it sounds, that even children are delighted to hear, and the unlearned come to drink, and when filled burst forth in singing....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)