In fa-yā ʿajaban, yā is formally the vocative particle, but the expression principally conveys amazement: “How extraordinary!” ʿAjaban appears in the accusative within this exclamatory construction.
Kūrihā names the camel saddle or riding gear, with -hā referring back to the poet's mount. Al-mutaḥammal describes the equipment as having to be carried or transported.
Once the animal is slaughtered, its saddle no longer rests on its ordinary bearer. The poet turns this practical oddity into an expression of astonishment and a distinctive detail of the remembered day.