Psyllium husks are the seed hulls of the plant Plantago. This plant is farmed extensively in India. It is ground to make mucillage, a fiber powder that becomes a sticky gelatinous mass when mixed with water.
In horses, psyllium is primarily used to reduce sand accumulation in the intestine. The idea is that the gelatinous mass binds to the sand and helps it move out of the intestine.
There are conflicting reports about the effectiveness of psyllium for the removal of sand from the equine intestine, versus the effectiveness of hay diets alone. My personal experience is that psyllium husks are effective at removing sand, especially when administered in large doses through a nasogastric tube.
Psyllium is usually fed intermittently to horses that are at risk for sand accumulation. We recommend 1 week out of each month. Psyllium is commercially available as a horse supplement, either in powder or pelleted form. The equivalent for human consumption is Metamucil®.