While horses do need to drink plenty of water, flavored water can be harmful. The artificial sweeteners in flavored water can upset a horse's digestive system, causing colic and other health problems. Additionally, the flavors in flavored water can be too strong for a horse's sensitive taste buds and may discourage them from drinking as much water as they need.
It is best to provide horses with fresh, clean water at all times. If you are concerned that your horse is not drinking enough water, you can try adding a small amount of apple cider vinegar or salt to the water. This can help to make the water more palatable and encourage your horse to drink more.