Well for starter an officer SWAT officer must wear its uniform to be considered in function. This is a case of fear vs thread. The ponies is the show shown no resistance whatsoever and were terrified. They haven't recognized Fluttershy until she shown herself. So, Nightmare Night or not they could have been on defensive, protect each others especially after all those dangerous creatures they faced in the past. Remember the fights against the changelings? Of course Twilight would have not hurt her friend if she known it was her.



If you were attacked by your friend wearing a full body costume that so you CAN'T recognized, would you be in defensive despite being in Halloween and knowing that its scary for fun. But what about it goes beyond play and you get scared, you would try to defend yourself do you? I know your friend could not, but some can become jerks and this. You would try to hold or stop him or her and others for help? That was I meant for Twilight to do.