Onions, garlic, leek, shallot and chive are from the Allium genus
of plants, which are toxic to horses if ingested.
The plants contain propyl disulfide which can lead to the formation of oxygen free radicals that damage the membrane of red blood cells, causing cells to release haemoglobin.
The haemoglobin binds to the inside of the red blood cells and forms Heinz bodies. These damaged red blood cells are removed from circulation which causes the horse to become anemic; the specific condition is called Heinz hemolytic anemia.