Julmūd denotes a large, weighty rock; ṣakhr identifies the stone material. In ḥaṭṭahu al-saylu, al-saylu is the subject, ḥaṭṭa the action of sending something downward, and the object pronoun refers to the rock.
Min ʿal indicates descent from above. The point is not simply that a horse is “hard like stone.” The scene is a massive rock hurtling downward in floodwater, bringing speed, irresistible power, and sudden movement into one comparison.
The simile's relevant qualities are acceleration, force, and velocity. Recognizing them explains why the comparison follows the four quick-change descriptors of the previous hemistich.