كُمَيْتٍ يَزِلُّ اللِّبْدُ عَنْ حَالِ مَتْنِهِ
كَمَا زَلَّتِ الصَّفْوَاءُ بِالْمُتَنَزَّلِ
“A dark reddish-bay horse whose saddle-pad slips from the slope of its back, as something slips down a smooth rock.”
Kumayt is a specific horse-color term, broadly a dark reddish or bay coat. Yazillu al-libdu describes the saddle pad or felt slipping away; this is an image of the back's smooth, streamlined, sloping surface.
Al-ṣafwāʾ denotes a smooth rock. The simile transfers the feature of a slick surface to the horse's back, not the idea of lifeless rigidity. The poet celebrates a muscular, well-formed frame without bulky irregularities.