The first edition of his most famous work was published by himself and didn't carry his name. Instead in the body of the text he described himself as -
"X, an American, one of the roughs, a kosmos, disorderly, fleshly, and sensual, no sentimentalist, no stander above men or women or apart from them, no more modest than immodest."
He kept adding and revising this work till his death in 1892, in Camden NJ.
One of his popular poems was written on the death of Y -
O Captain my Captain! our fearful trip is done;
The ship has weather’d every rack, the prize we sought is won;Recently, Levi's used one of X's poems in their advert -
"COME, my tan-faced children, Follow well in order, get your weapons ready;
Have you your pistols? have you your sharp edged axes? Pioneers! O pioneers!"
The video can be found at this link. http://goforth.levi.com/downloads
Try not to scroll down as that would give away the answer !
Id X and if possible, Y ?