Qāʾiman is an active participle in the accusative and, with bāta used as a deficient verb, serves as its predicate (khabar): the horse remained standing through the night. Ghayra mursal further qualifies that condition: not let go. Mursal is a passive participle of Form IV *arsala*, “to send or release.”
The contrast matters: qāʾim describes what the horse is doing; mursal concerns how it is treated. One expresses standing, the other release or non-release.