Ḍalīʿun is an adjectival description of the horse already introduced. Idhā opens a circumstantial time construction, while istadbartahu is a past-tense verb containing the subject and object pronouns: “you viewed it from behind.”
Sadda farjahu supplies the action witnessed in that setting. Bi-ḍāfin introduces the means by which the gap is filled: the full tail.
Laysa bi-aʿzalin denies a defect, with the additional bāʾ reinforcing the negation in the predicate.