a 30 gallon bladder tank will do all you need,
what you want is a "stock tank" that you fill, and is the water reserve for the animals, you want a tank that will hold enough that they can get a drink before the tank is empty, prefable less than a 1/3 or 1/2 empty, and then put a float valve on the tank to automatic refill it,
I have two 8 foot galvanized tanks for the cattle in the summer time and usually run up to 100 animals on the grass land,
my windmills pump about 2 gallons a min, so it is not the wells capacity or the wells pressure tanks, but the stock tanks that you need the reserve in.
if you have 20 gallons a day per horse I think would be plenty,
http://ohioline.osu.edu/anr-fact/0012.html
Horses, 10-15 gallons
http://www.agr.gc.ca/pfra/water/waterreq_e.htm
Horses - winter 8 gallons, summer 12 gallons
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Also I have one water line that goes into the pasture, a 1/2 mile and will provide water at good pressure and gpm as if it was at the house,
and I have one windmill yes windmill that pumps 265 up out of the ground and then horizontal 3000 feet to a storage tank, with a slight elevation on the horizontal of approx 60 additional feet of elevation,
On the distance run have a large enough pipe to reduce the flow restriction, (there are charts to show friction lost), and you can pump water for literaly miles and miles , jsut like city water does.
your only doing it on a smaller scale,
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One other item if your wanting to flush the pipe out for fresh water, it may take a long time, you should look up the gallons the pipe holds pre foot, and then the gallons per min your well pumps, and you can estimate the approx, time it will take to flush the pipe, if your coming out of a hose bib, then measure the gals of water you get out of it, it may take 30 Min's to an hour, or more to flush the pipe with jsut fresh water.
I know the first time I filled the pipe from the windmill it took hours to reach the end of the pipe.