Yucca Schidiger, Yucca spp.
There are over 40 species of Yucca. The one that is mainly used medicinally is Yucca Schidigera.

Besides being popular for arthritis, Yucca is known for its blood purifying, kidney cleansing and liver cleansing ability. It has also been used for; diabetes, urinary problems, to prevent blood clots, to help with indigestion, constipation, to stimulate the appetite and as a diuretic. It may be used topically for skin problems.
Yucca may swab as a prohibited substance in competitions. Large doses over a period of time may cause gastrointestinal upset, although generally it appears to be very safe. As always, seek veterinary advice about any problems with your horse.
Be aware that Agave lecheguilla, another species of Yucca has caused hepatotixicosis in grazing animals, so be careful of the type of Yucca you use and allow animals to graze on.