Apple cider vinegar may benefit horses in several ways, including acting as an insect repellent and a skin conditioner when applied topically, and potentially aiding digestion and appetite when added to feed. It can also help remove soap residue from laundry used for horses and may assist in balancing pH levels in their diet, although some benefits lack scientific proof.  
For skin and coat
Insect repellent: Diluted apple cider vinegar spray can be applied to a horse's coat, mane, and tail to help deter flies and other biting insects. 
Soothing and moisturizing: As an astringent, it can help soothe itchy skin and improve coat condition. 
Cleaning: It can be used to wash blankets, saddle pads, and other gear, helping to remove soap residue and condition the fabric. 
For diet and digestion
Palatability: Adding it to feed can encourage picky horses to eat, especially when traveling or when their grain has changed. 
Digestion: Some believe it can help balance the acid/alkaline pH levels in the diet and support digestion, though scientific evidence is limited and more acidic diets could be problematic for horses with ulcers. 
Mineral support: It may contain trace minerals that play a role in nerve and muscle function. 
Other potential benefits
Respiratory support: Some believe it can support respiratory health. 
Hoof health: There are claims it may assist in breaking down excess calcification on hooves. 
Important considerations
Introduce gradually: When adding apple cider vinegar to your horse's diet, start with a small amount and gradually increase it to the desired amount to ensure they accept the taste and there are no negative digestive effects. 
Lack of scientific evidence: Many purported benefits of apple cider vinegar for horses have not been scientifically proven. 
Potential side effects: Excessive amounts could disrupt gut function. Do not use it on horses with or prone to gastric ulcers. 
For external use, dilute first: Always dilute apple cider vinegar with water before applying it to your horse's skin to prevent irritation