Either that or he's starting to grow horse hair

And also interestingly, most mammals have the same electrolyte needs as humans. Some need more or less water (esp desert animals), but the balance of the electrolytes themselves (sodium vs potassium vs magnesium etc) are a lot more similar than you would think. The safe water ratio for oral intake is pretty wide too because the 90% of the kidneys job is to rebalance that, so as long as the electrolytes are balanced in relation to each other you're fine. So for Gatorade the optimal electrolyte level is actually about half (I usually dilute it 1:1 with water) but most healthy humans can easily tolerate it as-is (but it's still only better than water if you're losing a lot of water to illness or exercise).

When I looked up the normal range for the metabolic panel of a dog, the electrolytes were almost identical to a humans. I remember there being a theory that it relates strongly to when in the evolutionary chain we left the oceans, which is pretty similar for most mammals. Assuming this is true of horses, I wouldn't think this would be any more harmful than Gatorade or pedialyte, and even beneficial in some cases (significant fluid loss to illness or exercise).