“And he mounted upon his ass.” The ass was the type of the body of Christ, upon whom all men, resting from their labours, are borne as in a chariot. For the Saviour has taken up the burden of our sins. Now the angel who appeared to Balaam was the Word Himself; and in His hand He held a sword, to indicate the power which He had from above.
Source: Fragments from the Lost Writings of Irenaeus (New Advent)