Servius, Commentary to Vergil’s Aeneid 4.196
Iarbas: the son of Jupiter Ammon. When Bacchus (or, as others say, Hercules) was attempting an attack on the Indians, and was leading his army through Libya (that is, through Xerolybia), he was worn out by thirst and so begged for help from his father Jupiter. Jupiter showed him a ram, following which Bacchus arrived at a certain spot, in which the ram dug out some earth with its foot, and from this place a fountain flowed forth. Whence it happened that a temple holding a statue with a ram’s horns was established for Jupiter Ammon, who derived his name from the ‘sand’. This story has been contrived either because the god’s responses are sufficiently intricate, or because the Libyans call the ram ‘Ammon’. Others say that this Ammon was born in another place, where there had been a lone sheep, and where he was discovered by some nearby people. He was believed to have been born of Jupiter and the sheep, and named Ammon from the sand, because that is the nature of the ground there. Others say that there is a place between Cyrene and Carthage in which pastors found a boy distinguished by ram horns sitting in the sand and delivering prophecies. When they picked him up, he was silent; when they set him back down, he began to speak again. Soon, when he withdrew from the sight of humans, he was believed to be a god. From that time, they began to cultivate Jupiter with the name Ammon because he had been seen in the sand.
IARBAM: filium Iovis Ammonis. Liber, vel ut alii dicunt, Hercules, cum Indos peteret, et per deserta Libyae, hoc est per Xerolibyam, exercitum duceret, fatigatus siti Iovis patris imploravit auxilium: cui ille arietem ostendit, quem secutus ille pervenit ad locum quendam, in quo aries terram pede suo scalpsit, e quo loco fons manavit. unde factum est, ut Iovi Ammoni, ab arenis dicto, templum cum simulacro cum cornibus arietinis constitueretur: quod ideo fingitur, quia satis eius sunt involuta responsa, aut quia Libyes Ammonem arietem appellant. alii hunc Ammonem in loco natum, ubi sola ovis fuerat, a finitimis inventum dicunt, creditumque ex Iove et ove natum appellatumque Ammonem ab arena, quia ibi tale solum est. alii inter Cyrenas atque Carthaginem locum tradunt fuisse, in quo pastores puerum arietinis cornibus insignem, in arena sedentem ac vaticinantem deprehenderunt. hic sublatus tacebat, repositus loquebatur: mox cum e conspectu hominum subito recessisset, creditus est deus: unde Iovem ideo ibi colere coeperunt, nomine Ammonem, quod in arena fuerat visus.