"The herded wolves, bold only to pursue; The obscene ravens, clamorous o'er the dead; The vultures to the conqueror's banner true Who feed where Desolation first has fed, And whose wings rain contagion; how they fled, When, like Apollo, from his golden bow The Pythian of the age one arrow sped And smil'd! The spoilers tempt no second blow, They fawn on the proud feet that spurn them lying low.
Shelley is done mincing words here. People are acting beastly, and he's real mad about it. He calls those who criticize Keats "wolves," the type that are only brave when they are chasing their prey.
He also calls them "ravens" and "vultures," who feed on the dead, where "Desolation first has fed." Their cries are also like those of birds, who are "clamorous" (noisy) around a dead body.
They flee, though, when Apollo (Greek god of Prophecy) shoots at them. These fortune-telling arrows (described as "Pythian"—relating to one of Apollo's priestesses) that scare away the birds are symbolic of the realization that Keats was actually a great poet. In fact, once the critics realized he would be famous, they "fawned" (flattered and praised) Keats.
Yep, we told ya Shelley was mad. He hates that the people that once criticized Keats changed their tune when he became more respected and famous after his death.