The BLM uses freeze marking to identify captured wild
  horses and burros.  It is permanent, unalterable and
  painless.  Freeze marking is applied on the left side of the
  neck and cannot be altered.

  The technique is simple and completely painless to the
  animal.  The left side of the neck is shaved and washed
  with alcohol, and the mark is applied with an iron that is
  chilled in liquid nitrogen.  The hair at the site of the mark
  will grow back white and show the identification number.

  The following graph illustrates how to read a freeze mark.