I have given my mare Prilosec at times to ease her tummy. I usually begin this when I notice she is a bit girthier than normal. She is also on a supplement of Raspberry leaves. You can buy these leaves on herbspicetea.com cheaply, and by the lb. If you look at the expensive supplement Mare Magic, it is one of the top ingredients. Furthermore, I also give her chaste berries, which need to be ground, but can also be found on the website I listed. They also help regulate a mare's hormones, and can be done in cost-effective way with definite results. My mare is not Mare-ish in any sense of the term, but I do find that because she is so level-headed and willing most days, I became really sensitive to noticing the small changes her cycles caused. Now that she is on the supplements, she is regular, and more even-keeled.
You can google Prilosec, chaste, and raspberry leaves for horses and draw your own conclusions. I prefer natural solutions for her, but some people have suggested regumate, which I would never use on my mare.