This works because the glow is coming from radioactive decay which the pony then converts into useable sugars through photosynthesis. I'm sure she draws additional nutrients from the soil. No violation of the conservation of energy here, this is a nuclear powered pony! It's perfect!
The horse would be unable to survive on it's own light, as it would require energy to produce it's own glow, and it's glow would not radiate as strongly as the suns light, thus producing less energy for photosynthesis, should the sun disappear this horse would die as it's own energy dwindles from not having a source of external energy to fuel it's body.
Interesting concept but there is one problem. If this is a bioluminescence this horse MUST eat to produce the necessary chemicals that allow her to glow. So this generating energy from light is only the secondary support "life source".