“Then the horse.” Although some scholars see this verse as the Canaanite army trying to desperately escape, it seems to be more of a summary of the attack, with the hoofs hammering the ground in the attack. By the time of the escape, the ground was soaked and there would not be the hammering of the ground. The word “then” at the start of the sentence does not demand strict chronological order.
“hammered.” There is a wordplay here with the word “hammer” because it is the same root as in Judges 5:26 with the hammer that Jael used to kill Sisera.
“because of the galloping.” The galloping caused the hoofs to hammer the ground.
“mighty ones.” This word could refer to the horses, or the riders, or the combination of the horse and rider as a “mighty one.” Most scholars think it just refers to the horses, but there are verses in Scripture where the mighty ones are people.