Charles Ellicott Commentary Deuteronomy 33:17

"The firstling of his herd, majesty is his; And his horns are the horns of the wild-ox: With them he shall push the peoples all of them, [even] the ends of the earth: And they are the ten thousands of Ephraim, And they are the thousands of Manasseh." — Deuteronomy 33:17 (ASV)

They are the ten thousands of Ephraim, and they are the thousands of Manasseh.—Rashi refers this to the ten thousands slain by Joshua, the Ephraimite leader, and the thousands slain by Gideon, who was of the tribe of Manasseh. He expounds nearly the whole of the verse in reference to Joshua and the conquest of Canaan.

There is an obvious similarity in the song of the Israelitish women after the defeat of the Philistines, “Saul has slain his thousands, and David his ten thousands.” The people “pushed to the ends of the earth” are taken to be the thousands and ten thousands of conquered Canaanites and Midianites. For a similar metaphor, see 1 Kings 22:11.

Otherwise, the ten thousands of Ephraim and the thousands of Manasseh would be the two-horned power of Joseph. (Daniel 8:20 for a simile of the same kind.)