There is a fair amount of research on cinnamon in people. Cinnamon does have some evidence as an anti-diabetic nutrient. In people and laboratory animal studies, it appears to reduce the rate at which glucose is taken up and reduces the risk of blood glucose spikes and improves cellular glucose regulation. As far as I am aware, there are no studies on the efficacy of cinnamon in horses. In people, the dose is 1-6g per day. This would equate to 7 to 42g per day in a 500kg (1100lb) horse. Cinnamon does contain a liver toxin called coumarin which can cause liver damage in high doses. Cinnamon tea reduces coumarin uptake as the coumarin remains in the sediment not in suspension.
Cinnamon safety data sheets refer to risk of allergic skin reactions, respiratory irritant, suspected of being mutagenic and “may cause cancer”.
Cinnamon does appear on the Feed Materials Register and so companies can include cinnamon in feeds and supplements. https://www.feedmaterialsregister.eu/register
An EFSA panel in 2021 concluded that in horses, cinnamon should be safe when added to feed at a rate of 60mg/kg complete feed. Taking for example, 3kg of complete feed a day, this would only equate to 180mg or 0.18g of cinnamon – way below the amount that is likely needed to have some degree of efficacy in horses based on human studies. https://pubmed.ncbi.nlm.nih.gov/34934461/
Companies selling cinnamon-based products for horses either in supplements, as pure cinnamon or in feeds have feeding rates equivalent to 15-20g/500kg horse (1100lb)per day.
I am not aware of any data to show that feeding horses 15-20g of cinnamon per day is either effective at improving blood and tissue glucose regulation nor that this is safe to do so. Evidence of efficacy in laboratory animals and people does not necessarily translate to horses. Until there is more evidence of its efficacy and safety in horses I could not recommend that it be used.