Garlic, if fed at high enough doses for a long enough period of time, can cause a horse to become slightly anemic.
"There is a toxic element in Garlic
called N-propyl disulfide which can change an enzyme within a horse's red blood cells
, it depletes the cell of a chemical known as phosphate dehydrogenase - whose task is to protect the blood cells from damage caused by oxidation When the level of phosphate dehydrogenase gets too low the hemoglobin in the blood cell oxidizes and forms a bubble. This is seen as being deformed as it passes through the spleen and is removed from the bloodstream. If the blood is consistently poisoned by N-propyl disulfide contained in a garlic supplement more red blood cells are removed and the horse
may slowly become anaemic
Researchers fed a healthy horse
1 lb of onion tops, which also contain N-propyl disulfide, over an 11 day period. By the 11th day the horse
had lost almost 60% of his red blood cells and was severely anaemic!
Some vets claim that the toxic effects of garlic are gradual - a low dose fed on a regular basis can result in mild anemia.
However it may be that commercially prepared equine garlic supplements
are not dangerous as the active ingredient -allicin - is often destroyed during the manufacturing process when heat dried.
The benefits of garlic
are many and if you do decide to feed a garlic supplement
just be careful not to overdo it."
From Feeding Garlic to Horses - benefits and dangers
. Bold from quote not mine.