What is the allusion used in the poem "My Last Duchess" by Robert Browning?

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.