Midst others of less note, came one frail Form, A phantom among men; companionless As the last cloud of an expiring storm Whose thunder is its knell; he, as I guess, Had gaz'd on Nature's naked loveliness, Actaeon-like, and now he fled astray With feeble steps o'er the world's wilderness, And his own thoughts, along that rugged way, Pursu'd, like raging hounds, their father and their prey.
More people show up, too, but apparently they aren't too important. The speaker describes them as "others of less note." To him, poets are the most important people, apparently.
Another mourner arrives, alone. Shelley uses a simile to compare the frail man to the "last cloud" of a storm.
But he has some power; he still makes "thunder" (a "knell" is a sound that announces something, usually death). Perhaps he is another poet, one who came late to mourning Keats but who still has something to say about it.
The nameless mourner is compared to Actaeon of Greek myth, who died after being torn apart by dogs. Could this be as symbol of another poet who died after facing harsh criticism? Shelley doesn't give us his name, so we can only guess.