a mare and a stallion are NEVER EVER safe to be put together unless you want a foal. Heat or no. You have no idea what's going on when you aren't around, and I'd bet money she's been bred, whether or not she acts snappy when he comes near. A stallion's life mission is to breed, and he WILL do it. Seriously - since I've joined this forum there's been SO MANY "Oh, they were only together for a day," or "Can't be pregnant, she's not in heat/cycling," even "They're separated by a fence!" And BINGO, horse is pregnant. It's made me come to the conclusion that horses WILL have at it if you give them half a chance - and you've given them MORE than half a chance.