וַֽיִּרְמְסֶֽנָּה - Qal imperfect with waw consecutive 3rd person masculine singular of רמס
The senses in the MT (generated with Logos Bible Software)
Apparently the issue is the verb is singular. Thus literally "he trampled her." The Septuagint (LXX) has plural.
καὶ συνεπάτησαν αὐτήν (ending of 2 Kings 9:33, LXX)
The NASB95 has the most literal translation.
He said, “Throw her down.” So they threw her down, and some of her blood was sprinkled on the wall and on the horses, and he trampled her under foot.
There is a struggle how to make this clear. "He" means Jehu, but the reference to horses sounds like Jehu was in a chariot. It is doubtful that Jehu go out of his chariot to personally stomp on Jezebel, so he rode over her with the horses and his chariot makes sense. But, this is interpretation rather than translation.