If your girlfriend ever called you a caveman, it is pretty sure she did not mean it because you wore a fancy leopard-print dress with a strap over one shoulder and uneven hem, with black bushy wig and beard.
But, what a great idea for a stag do fancy dress.
The costume would not be complete other personalized bits stuck in the hair or beard, maybe a stick or chicken bone would do the trick, or bird feathers, bones, your girlfriends garter…

Of course, the look will get more authentic as the stag do progresses and more beer is consumed.

A word of warning: do not attempt to drag your girlfriend by her ponytail; modern girls don’t go for that.
 

The Caveman Fancy dressgoes well with the " Caveman kick it old school" or "A Simple Man, with Simple Problems" Stag DO Group T-Shirts

 

or have the everyone attending stag do wear the Caveman fancy dress, and be the cavemen, now that would cause a stir. and ensure the stag do is never forgotten.