Hylas

In lines 43-44, Silenus briefly refers to the story of Hylas, a handsome youth and the son of Theiodamas, the king of the Dryopes whom Hercules had killed. After killing the king, Hercules spared Hylas and took him as his companion aboard the Argo. Theocritus devotes a whole poem to him (Idyll 13), and Apollonius of Rhodes tells his story in Argonautica 1.1207ff.

Once, when Jason and his crew had made a stop during their journey, Hercules sent Hylas off to fetch water from a spring. But the water nymphs of that spring were struck by the youth's beauty and pulled him down beneath their waters. Hercules searched far and wide for Hylas, shouting his name in desperation. According to Theocritus, only a thin voice escaped the water in reply (ἀραιὰ δ᾽ ἵκετο φωνὰ / ἐξ ὕδατος). Hercules, profoundly aggrieved by the loss of his lover, abandoned the voyage; and the Argo went on without him. Theocritus sets the story within a context of a lover's grief, and in the end Hylas joins the number of the Blessed (οὕτω μὲν κάλλιστος ῞Υλας μακάρων ἀμιθρεῖται). 

The Oxford Classical Dictionary (2nd ed.) makes a connection between the Hylas story and an ancient local custom via a reference to Apollonius, who says that in his own day the people of Cios still looked for Hylas (1.1354). The OCD notes that "ritual search for a deity, perhaps of vegetation, is not unfamiliar in the Greek world..." 

I have been unable to find a certain etymological link for Hylas' name, though possible connections are to (1) ὑλάω, 'to bark, howl'; and (2) ὕλη, 'wood, forest, woodland, etc.'

Vergil also mentions Hylas once in Georgics 3.6 

Hylas and the Nymphs by John W. Waterhouse