Why can the feminine singular «عَاكِفَةً» describe «الْخَيْلَ»?

ʿAmr ibn Kulthūm: Address to Abā Hind, Tribal Pride and the Mill of War (Verses 23–38)

• Al-khayl must always be masculine singular
• Al-khayl is collective and may take a feminine singular descriptor
• ʿĀkifah is an imperative
• ʿĀkifah is a genitive complement