Anal refers to the anus and the surrounding perineal tissues — the terminal opening of the equine digestive tract through which feces are expelled. In horses, the anus lies in the perineal region beneath the tail, dorsal to the vulva in mares and above the scrotum or sheath in males.
The anus is a clinical landmark in several examination contexts. In mares, the relative position of the anus and vulva (perineal conformation) is used to assess reproductive soundness: a tipped vulva with the anus positioned cranially creates a pneumovagina risk, where air and fecal contamination enter the reproductive tract. This conformation defect is corrected surgically with a Caslick’s procedure. In rectal palpation — a standard technique for evaluating colic, reproductive status, and internal organ size — the examiner passes a lubricated arm through the anus into the rectum.
Perianal abnormalities in horses include melanomas (especially in grey horses, where subdermal melanoma commonly presents at the perineum and anus), rectal tears during examination or foaling, and anal sphincter laxity secondary to neurologic disease. The anus is examined as part of the perineal region in a standard prepurchase evaluation. For anatomical context, see the full external anatomy reference. Colic with straining can be confused with rectal prolapse, which may involve the anal sphincter; see how to distinguish straining from colic.
Further reading: Anal canal on Wikipedia; gonads and genital tract of horses from the Merck Veterinary Manual, covering perineal anatomy.