‘The winged boy I knew;But who wast thou, O happy, happy dove?’ is an example of-

Allusion is an implicit or indirect reference to another work of literature, a historical or mythical person or event.
Examples:
‘The winged boy I knew;
But who wast thou, O happy, happy dove?’
(Keats: “Ode to Psyche”)
The “winged boy” is an allusion to Cupid, the god of love.
''Not half so fixed the Trojan could remain,
While Anna begged and Dido raged in vain'' is another example of allusion.
(Pope: The Rape of the Lock)
Source: An ABC of English Literature - Dr. M Mofizar Rahman.