Charles Ellicott Commentary Genesis 49:16-17

"Dan shall judge his people, As one of the tribes of Israel. Dan shall be a serpent in the way, An adder in the path, That biteth the horse`s heels, So that his rider falleth backward." — Genesis 49:16-17 (ASV)

Dan. —Turning to the sons of the handmaids, it was necessary to assure them of an independent rank among their brothers. The four tribes descended from them always held an inferior position, but Jacob, by his words to Dan, prevented them from ever becoming subject states. Then, playing on the name Dan (a judge), he says that Dan will judge his people as a distinct and separate tribe, possessing all the rights of self-government and tribal independence that this rank implied.

It also seems that Dan’s symbol was a serpent. From this, Jacob prophesies that although Dan was too weak a tribe to take the foremost place in war, the tribe would still have military importance. This was especially true in the days of Samson. The word translated adder is more precisely the arrow-snake, which lies in wait in the “path,” a narrow track, and springs upon its prey as it passes. A horse bitten in this way would rear and throw its rider, who would then be in the power of his assailant.