John Gill Commentary Deuteronomy 2:11

"these also are accounted Rephaim, as the Anakim; but the Moabites call them Emim." — Deuteronomy 2:11 (ASV)

Which also were accounted giants, as the Anakims
Because of their bulky size and tall stature; or, "the Rephaim they were accounted, even as the Anakims;" they were reckoned Rephaim, a name for giants in early times, even as the Anakims were; see (Genesis 14:5).

but the Moabites called them Emims ;
to distinguish them from the Rephaim; so that it seems this name of Emims was not originally their name, but they are called so by a prolepsis, or anticipation, in (Genesis 14:5) since they had it from the Moabites, a people of a later date.