These are Isildur and Anárion, that guarded the passage of the Great River Anduin. They marked the ancient northern border of Gondor.
‘Fear not!’ said a strange voice behind them. Frodo turned and saw Strider, and yet not Strider; for the weatherworn Ranger was no longer there. In the stern sat Aragorn son of Arathorn, proud and erect, guiding the boat with skilful strokes; his hood was cast back, and his dark hair was blowing in the wind, a light was in his eyes: a king returning from exile to his own land.
‘Fear not!’ he said. ‘Long have I desired to look upon the likenesses of Isildur and Anárion, my sires of old. Under their shadow Elessar, the Elfstone son of Arathron of the House of Valandil Isidur’s son, heir of Elendil, has nought to dread!’