2 When rancour does with rage him once engore,
once > [once and for all, finally] engore > gore, wound deeply; hence: infuriate
3 Forgets with wary ward them to await,
ward > guard await > [keep watch for]
4 But with his dreadful horns them drives afore, 5 Or flings aloft, or treads down in the floor,
floor > ground (catachresis prompted by the rhyme)
6 Breathing out wrath, and bellowing disdain,
disdain > wrath
7 That all the forest quakes to hear him roar:
That > [So that]