We don't have synonyms for dead horse yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are deadhorse, dead house and deadhouse.
(n) :
(dated, nautical) The period of work on board ship for which the seamen have been paid in advance (usually a month's wages), the end of this term being celebrated by parading a straw horse about the decks.
(n) :
(dated, nautical) The period of work on board ship for which the seamen have been paid in advance (usually a month's wages), the end of this term being celebrated by parading a straw horse about the decks.