Tafsir of Al-Adiyat 100:3

Fa-al-mughirati: Derived from aghara upon the enemy, meaning to attack them suddenly with horses for the purpose of plunder, killing, or capturing. Thus, raiding is a quality of the horsemen, and attributing it to the horses themselves is either figurative or by way of estimating an implied genitive (muḍāf); the underlying meaning being: "The ones whose owners raid the enemy upon them." It is also said that the attribution is because the raiding occurs by means of them.

Subhan: That is, at the time of dawn. It is in the accusative case as an adverb of time (ẓarf). This was the customary practice in raids; they would travel by night so that the enemy would not perceive them, and they would attack in the morning so that they could see what they were taking and what they were leaving behind. They used to be emboldened by this, as in the poet’s saying:

My people, who brought morning upon the enemy at the day of al-Nakhil, with a persistent raid.