Ḥamaytu is first-person past from ḥ-m-y: ‘I protected.’ Taḥmilu is imperfect from ḥ-m-l: ‘it carries,’ describing the continuing state of the mount within a past episode.
Wa-laqad emphasizes the recollection, while idh ghadawtu locates the action at departure. Alternating verb forms conveys the event together with its unfolding setting.