Horse Nutrition, Bulletin 762-00, Protein
"High-protein rations have been reported to cause kidney damage in horses. This is an untrue old wives tale brought about because a horse on a high-protein ration urinated more, and the smell of the urine was stronger due to the excess urea being passed."
The Horse | High-Octane Diets
"Pagan also notes that total grams of protein are what count, not percent or concentration."
This is the ONLY "bad" thing about feeding too much protein, as stated in the article above: "Excess protein can lead to high levels of urea, which, when excreted in the urine, is converted to ammonia by bacteria in the stall bedding and floor. Too much protein also causes the horse to drink more water. Combined, these factors mean that a horse on a high-protein diet will urinate more, creating more ammonia in his environment. For a stabled horse, that can spell respiratory distress."
"There is no data, either scientific or otherwise, which suggests that protein intake has anything to do with mental attitude."