Robert Browning ends his poem "My Last Duchess" with an allusion to Neptune, the Roman god of the sea. Duke Ferrara, who speaks during this dramatic monologue, is giving a tour of his home to the agent of a Count whose daughter he intends to make his next Duchess. The poem begins with him showing the painting of the Last Duchess, presumably in an art gallery section of his home. As they go downstairs, he points out the bronze statue of the Roman god, which he commissioned from a fictitious sculptor named Claus of Innsbruck. Beginning and ending the poem with the Duke showing off his artwork brings the poem full circle and emphasizes his role as a Renaissance aristocrat—wealthy, pretentious, and arrogant.
The choice of "Neptune taming a seahorse" is interesting because it can easily be interpreted as the relationship between the Duke and his last Duchess. The Duke views himself as a god. However, for Neptune, the god of the sea and horses, to be taming a seahorse is ironic. A seahorse is a small, inconsequential animal not capable of fighting back. In the same way, the Duchess was powerless under the mighty hand of her husband, who tried to dominate her slightest move, even the smiles she gave people. Just as it would not have been a truly impressive feat for an immortal to hold sway over a harmless sea creature, so it should have been beneath the Duke to dominate, abuse, and do away with his last Duchess.
The poem, "My Last Duchess" by Robert Browning is a dramatic monologue in which the narrator alludes to the Roman god Neptune, "Notice Neptune, though,/taming a sea-horse, thought a rarity." This is a significant allusion in a couple of ways.
Neptune was a God of the sea, but Poseidon seemed to be much more predominant. In fact Neptune became more noted for ruling horses and horse racing than he was for ruling the sea. Thus the allusion to Neptune taming a sea horse makes sense with regard to the mythological reference.This is also significant because once again the Duke is leaping from women to works of art in the same discussion which indicates (as is supported throughout the poem) that he considers women to be objects such as art. The seahorse in the last referenced piece of art is perhaps the wife he couldn't really tame.