John Gill Commentary Jeremiah 51:21

"and with thee will I break in pieces the horse and his rider;" — Jeremiah 51:21 (ASV)

And with thee will I break in pieces the horse and his rider ,
&c.] Or, "have broken": meaning the cavalry of an army, wherein lies its chief strength: and with thee will I break in pieces the chariot and his rider ;
which were also used in war.