Virgil’s Aeneid places Lethe among the underworld rivers Aeneas reaches; souls drink forgetfulness there. A named river attested in that Roman katabasis, not Styx the oath-crossing and not a generic water of the dead.
from Virgil · Aeneid, 19 BCE (unfinished at the poet’s death)
Ovid’s Metamorphoses underworld speech names Lethe among the waters of forgetting beside Orpheus’s descent. A named Lethe in that poem, not a roman Lethe seat alone and not Acheron as the same river.