There is no evidence for the popular belief that Sable's horses arrived as survivors of a shipwreck.

The true story
A Boston clergyman, the Reverend Andrew Le Mercier, sent the first horses to graze on the island in 1737. Most of them were probably stolen by privateers and fishermen.
About 1760, Boston merchant and shipowner Thomas Hancock shipped 60 horses to Sable. These horses survived and became wild.

Artist's rendering of an Acadian homesite at Belleisle, N.S., before the Expulsion - watercolour by Azor Vienneau, Nova Scotia Museum collection.

But whose horses were they?
Between 1755 and 1763, Acadians were deported from Nova Scotia by British authorities. Hancock was paid to transport Acadians to the American colonies. The Acadians were forced to abandon all their livestock. It appears that Hancock helped himself to some of their horses and put them to pasture on Sable Island.