Dandelion 
Taraxacum officinale

    This deceptively simple and common plant is, in fact, a powerful medicine. It is diuretic, alterative, bitter and mildly laxative. It is deep-rooting and therefore brings up valuable minerals from the deep layers of the soil which are inaccessible to grass roots. Dandelion's reputation as a weed has seen its decline in many pastures, but some believe its establishment in horse pastures should be encouraged. Used in cases of liver, kidney, skin or heart problems, it has powerful beneficial effects apart from its basic nutrient value. It is also rich in potassium, a valuable supplementary benefit in is application as a diuretic. The root is a more powerful diuretic than the leaf, and can be obtained as a powder. The diuretic and alternative properties make it one of the valuable herbs to be given in cases of laminitis (founder). 
Notes: Many folks spend money for expensive treats that in most cases present no important nutritional value and often too much sugar. If you wish to give your horse some treat, pick a couple of Dandelions and give to him. Much healthier treat especially when you pick it with the roots. No need to wash it, just shake off the dirt and that will do. Do not pick it near polluted grounds, oil, gasoline and freshly or richly fertilized lawns.