Tahhan, a horse owned by Sheikh Mohammad Bin Rashid Al Maktoum, tested positive to 16-b Hydroxy-Stanozolol and Guanabenz after 120km rides in Bahrain and Dubai in January and February.
The testing was undertaken by the Sheikh's staff, and he voluntarily informed the FEI of the results on March 19.
The Sheikh said in an official statement that he was unaware of the administration of the substances, and condemned their use. However he also accepted that he was the "person responsible" for the horse.
He is also undertaking an internal investigation into the cases, and said he would share the information with the FEI. The Sheikh withdrew from a ride last weekend because of the cases.
Guanabenz is used to treat high blood pressure in humans. In horses, it is used to reduce the incidence or severity of exercise induced pulmonary hemorrhage (EIPH). Hydroxy-Stanozolol is a metabolite of Stanozolol, a synthetic anabolic steroid.