It’s an identification symbol burned into the hide of a wild horse or burro after capture, usually on the upper left side of the neck. The procedure is often carried out in the field before the animals are brought to BLM corrals for adoption.
The last four digits correspond to the number on the neck tag worn by the animal at adoption. Freeze marks can be hidden by manes and will change in appearance as the horse ages and as his winter coat comes and goes.
RELATED: The Mark of Authenticity.
Western Horse Watchers Association 