Jamieson, Fausset and Brown on Isaiah 51:20

20. head of all ... streets--(La 2:19; 4:1).

wild bull--rather, "oryx" [Jerome], or gazelle [Gesenius], or wild goat [Bochart]; commonly in the East taken in a net, of a wide sweep, into which the beasts were hunted together. The streets of cities in the East often have gates, which are closed at night; a person wishing to escape would be stopped by them and caught, as a wild animal in a net.