He's a good friend of William and Harry, and organised the royal stag do.
From the guardian.co.uk
The stag is a fallow deer, a non-native species kept at that time in royal parks for hunting.
From the en.wikipedia.org
They were photographed leaving a London nightclub together, and Middleton was snapped on a stag hunting expedition at the royal family's Balmoral estate in Scotland alongside Charles.
From the suntimes.com
Mithridates was reportedly guarded in his sleep by a horse, a bull, and a stag, which would whinny, bellow, and bleat whenever anyone approached the royal bed.
From the en.wikipedia.org
While the world's media may have missed out on William's stag do, innumerable other festivities-from pub promotions to house parties-are planned to coincide with the royal wedding itself.