Ibi cum eum somnus oppressisset, pastor quidam nomine Cacus, ferox viribus, boves pulcherrimos e grege rapere et praedam in speluncam suam abdere voluit.
ferox viribus? - mad with power? This doesn’t seem very intuitive. A shepherd driven mad with power seems a bit odd. Do I have this wrong…
Brutally strong, maybe, or brutal in respect to his strength. ferox is in apposition to Cacus (nominative), and viribus you could call an ablative of specification/quality/description.
In my part of the world, people say “fiercely strong” or “fierce strong” and “fiercely proud” or “fierce proud” and “fierce handsome”, with “fiercely” meaning “extremely”. Nostrâ in regione, ferociter vel ferox pro adprimè dicitur.