The following is based on Fallout Tactics and some details might contradict canon. |
Brahmin armor is a special encounter in Fallout Tactics. There's a brahmin guarded by four laser turrets. By defeating the turrets and the brahmin, you will find the 'brahmin armor' which allows the wearer to take the appearance of a brahmin (actually you can just sneak past the turrets but, if you don't destroy them you may not be able to leave - needs to be verificated).