I'm so sorry for your loss. I agree with some of the other people, it sounds like some kind of poision. It is so hard when they didn't have any signs to figure out what happened. So I would take inventory of everything in the barn that both foals have come into contact with, such as, hay, grain, beet pulp (check for sand or spoilage), water buckets or troughs, feed suppliments, vaccinations, wormer, pasture for weeds, tree leaves (red maple leaves could have blown into the pasture), bugs, birds (check the water trough for dead birds or animals), disenfectants, ect. Could anyone along your fence line have sprayed any chemicals near your pasture? I would keep the colt in the barn and away from everything until the results come back. I'll pray that you get results soon.