It's probably a brand as others have mentioned. There are a few types of branding, a TB will have a lip tattoo, but sometimes if a horse was bred at a praticular farm they will brand their horses with a regular "hot" brand or freeze brand.
A hot brand is basically a metal design on a handle that they get hot and then put it on whatever part of the horse, the other popular brand is the freeze brand.
"A freeze brand is the result of applying a very cold branding iron to the hide of an animal. The three most common coolants used to make a freeze branding iron very cold are: 1) A combination of dry ice and 99% alcohol, or 2) A combination of dry ice and acetone, or 3) Liquid nitrogen. A branding iron made specifically for freeze branding is made of different metal(s) than a "hot" branding iron so that it will retain the cold from the coolant more efficiently."