Why the diarrhea?? usually a sign of something more serious I wouldn't be mussing around trying out herbal remedies but getting horse seen by vet to see the cause.
After you know what your treating,vet may recommend Biosponge which is a natural/organic type product.
Asides from getting a consultation from the vet, oak bark is the best remedy against diarrhea there is. However, be sure to check this information, because oak species where you live can differ from those in my country, so better safe than sorry. The bark is made into tea that is mixed with feed.
HHH: An excellent equine astringent is white oak bark. Note: Oak acorns can be toxic if consumed by horses.
White oak bark (Quercus alba) is known for its astringent qualities and ability to strengthen capillaries. It has been used topically for sores and internally for diarrhea, hemorrhage, leucorrhea, bleeding and ulcerated gums.
* Try using a spray bottle to apply astringent herbs solution to remove debris without forcing dirt deeper into the wound.
* Marijke van de Water B.Sc., DHMS of Riva's Remedies calls white oak bark a “great astringent with an affinity for ligaments and tendons. It contains a fair number of nutrients as well.”
* Check out Riva's Remedies Happy Foot formula which contains white oak bark, along with other herbal supports.