My horses don't eat any of them as they tend to grow where they have pooed - which I'm pleased about as we eat them! I don't think an ordinary field mushrooms, parasol mushrooms etc will harm horses, but I am prepared to be enlightened.
if the flesh is pinky and they peel pick asnd make soup add to casseroles etc. or leave and let the horses trash them. its all to do with the Ph in the soil, type of fertilizer used and wether conditions! they can sprout up in 2 to 3 hours, best picked fresh, and a drizzle of rain makes them sprout. Enjoy