POOR OLD HORSE, as in Countess Richard's first reply, is taken (and much mauled) from the sea shanty of that name. This shanty was originally only sung after one month at sea, in a ceremony to mark the end of "paying off the debt" where sailors had pledged their first month's pay to some boarding master.
The lyric you are seeking comes from a totally different ceremony, that of broaching the first barrel of salt beef, after the fresh provisions had run out. It is more correctly known as THE SAILOR'S GRACE.

Stan Hugill gives:

Salt horse, salt horse, we'd have you know
That to the galley you must go.
The cook without a sign of grief
Will boil you down and call you beef,
And we poor sailors standing near
Must eat you though you look so queer.
Salt horse, salt horse, what brought you here?

Salt horse, salt horse, both near and far,
You're food for every hard worked tar.
In strongest brine you have been sunk,
Until you're hard and coarse as junk.
To eat such poor and wretched fare
Would whiten any nigger's hair.
Salt horse, salt horse, what brought you here?

Salt horse, salt horse, what brought you here?
After carrying stones for many a year
From Bantry Bay to Ballywhack,
Where you fell down and broke your back,
And after years of such abuse,
They salt you down for sailors' use.
Salt horse, salt horse, what brought you here?

These lyrics could vary from nationality to nationality, or even from ship to ship. But the story is told much the same.

There! You got it from the horse's mouth...