Albert Barnes Commentary Job 39:21

"He paweth in the valley, and rejoiceth in his strength: He goeth out to meet the armed men." — Job 39:21 (ASV)

He paws in the valley - The margin says, “or, His feet dig.” The marginal reading is more in accordance with the Hebrew. The reference is to the well-known fact of the “pawing” of the horse with his feet, as if he would dig up the ground. The same idea occurs in Virgil, as quoted above:

cavatque
Tellurem, et solido graviter sonat ungula cornu.

Also in Apollonius, Book 3, “Argonauticon”:

Ὡς δ’ ἀρήϊος ἵππος, ἐελδόμενος πολέμοιο,
Σκαρθμῷ ἐπιχρεμέθων κρούει πέδον.

Hōs d' arēios hippos, eeldomenos polemoio,
Skarthmō epichremethōn krouei pedon.

“As a war-horse, impatient for the battle,
Neighing beats the ground with his hoofs”

He goes on to meet the armed men - The margin says, “armor.” The margin is in accordance with the Hebrew, but still the idea is substantially the same. The horse rushes on furiously against the weapons of war.