Yakādu is a verb of approximation. With a following imperfect verb it suggests that an action is on the verge of taking place. Yakādu l-ṭarfu yaqṣuru therefore means that the gaze almost fails to encompass the horse’s full figure.
Compare:
Yakādu l-ṭarfu yaqṣuru — The eye nearly falls short.
Qaṣura l-ṭarfu — The eye fell short.
The first is more evocative: it conveys near occurrence and a heightened impression instead of reporting only a completed fact.