Adonais (often named "Adonis"), god of beauty, was a tragic figure in Greek mythology. Though desired by many, some gods were envious of his sex appeal and youth. One such jealous deity, Artemis—goddess of the hunt—sent wild boars after him in a fit of jealousy. The boars tore him apart, devastating his fellow gods and goddesses.
Even today, Adonais is seen as a symbol of the epitome of male youth and beauty. Shelley used him to symbolize the poet John Keats, who also died tragically young. But it wasn't just the circumstances of his death that made Adonais a good stand-in. Shelley believed that Keats was a poet with more talent than any other. The loss of such a man was particularly devastating to him, and so he wanted an appropriate symbol of loss that others would immediately recognize. And Adonais fit the immortal, heavenly bill.