A. This can potentially be very serious, and I would recommend calling your vet. Diarrhea (this bad) in horses is not very common, and usually a sign of another illness or disease parasites. She is quite young which also increases the risks and easier to become dehydrated. Especially if you haven't recently changed anything in her diet, and this has been for more than a day or so, please have her seen by a vet.
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