They shall roar together like lions
Some understand this of the Medes and Persians, and the shouts they made at the attacking and taking of Babylon; but this does not so well agree with that, which seems to have been done in a secret and silent manner; rather according to the context the Chaldeans are meant, who are represented as roaring, not through fear of the enemy, and distress by him; for such a roaring would not be fitly compared to the roaring of a lion; but either this is expressive of their roaring and revelling at their feast afterwards mentioned, and at which time their city was taken; or else of the high spirits and rage they were in, and the fierceness and readiness they showed to give battle to Cyrus, when he first came with his army against them; and they did unite together, and met him, and roared like lions at him, and fought with him; but being overcome, their courage cooled; they retired to their city, and dared not appear more; (See Gill on Jeremiah 51:30); they shall yell as lions' whelps.
Jarchi and other Rabbins interpret the word of the braying of an ass; it signifies to "shake"; and the Vulgate Latin version renders it, "they shall shake [their] hair"; as lions do their manes; and young lions their shaggy hair; and as blustering bravadoes shake theirs; and so might the Babylonians behave in such a swaggering way when the Medes and Persians first attacked them.