Dan shall be a serpent in the way, a viper by the path, that bites the horse’s heels so that his rider falls backward.
Here we have Jacob's prophecy concerning six of his sons. I. Concerning Zebulun (Gen 49:13), that his posterity should have their lot upon the seacoast, and should be merchants, and mariners, and traders at sea. This was fulfilled when, two or three hundred years after, the land of Canaan was divided by lot, and the border of Zebulun went up towards the sea, Jos 19:11.
Commenting on Genesis 49:13-21
I have waited for thy salvation, O Lord. Jacob finding his spirits faint and flag, stops and breathes awhile before he proceeded any further in blessing the tribes; and as he found he was a dying man, and knew not how soon he should expire, expresses what he had been thoughtful of and concerned about in time past, and still was; that he had been...
DAN--though the son of a secondary wife, was to be "as one of the tribes of Israel." (Gen 49:16-18) Dan--"a judge." a serpent . . . an adder--A serpent, an adder, implies subtlety and stratagem; such was pre-eminently the character of Samson, the most illustrious of its judges.