The following morning our three travelers run into these horsemen, who are led by none other than Éomer.
While the meeting is a bit hostile at first, eventually they warm up a bit and Éomer even offers them two horses after telling the story of Théoden's corruption by Saruman, which resulted in his own banishment.
Unfortunately, he also tells them that they left none alive when they slaughtered the orcs. Oh, and they didn't see any hobbits.
So Gimli, Aragorn, and Legolas ride to the pile of burning orc corpses and, while at first dismayed by the smoldering hobbit belt, Aragorn sees signs of their struggle in the earth.
They follow this path and realize Merry and Pippin escaped into Fangorn Forest.