Mild and stainless steel.
Update! This horse armour is missing. I rented the armour to a customer who evidently sold it and spent the money. The renters first name begins with a "P" I haven't been able to contact him but he does frequent joust competitions around the country. If you happen to see this horse armour please don't approach the person but rather contact me so that I can notify the proper authorities. If possible take a photo of the armour just so that I can be sure it is the harness I made. There are similar sets of horse armour out there and I don't want to cause any stress to someone who legitimately purchased theirs.
This horse armour was based on a composite war harness from the late 14th century. The original shaffron (head piece) had a long sharp spike pointed out from between the eyes like a unicorn. I decided to eliminate this spike for safety reasons. Severe injuries could be inflicted to a handler if the horse were to try to scratch its head on someone. For the comfort of our equine friends, I will expect be remaking this harness in aluminium sometime in the future.