A pastoral elegy is a hybrid form combining the conventions of the pastoral with those of the elegy. A pastoral is a work set in an idealized version of the countryside, populated by shepherds, shepherdesses, happy farmers, and benign nature, in which people getting lost in the forest and spending nights outside wearing city clothes never get hypothermia or even insect bites. The pastoral portrays its imagined countryside as a place of prelapsarian innocence, far removed from the vices of the cities.
An elegy is a funeral song or lamentation for the death of a beloved or admired person.
Thus a pastoral elegy is a lamentation for the death of a person (real or fictional) who rather than being described realistically is imagined within an idealized pastoral environment.
Some famous examples of the pastoral elegy in English are is Thomas Gray’s “An Elegy Written in a Country Church Yard”, Milton’s “Lycidas”, Shelley’s “Adonais”, and Matthew Arnold’s “Thyrsis” (1867)