Even back in the day (When i was working cattle 1980’s) , hot irons were used on cattle only. Nitrogen for horses. The nitrogen causes brands to be white ... shows up easy on horses. My donkey from blm land in nevada had a nitrogen brand. my QH mare out of oklahoma had a freeze brand. All my horses from Idaho have no brand.
funny thing. I had to get a brand inspector to give me paperwork for my 3 Fjord mares so I could travel to a show in wash and montana. He filled out the paperwork on one, took pictures. Then he looked at the other 2 and asked me why I cant use the same paperwork for all 3. No one can tell them apart.