While, yes, it may be a phrase from Act 3, Scene 1, line 273 of William Shakespeare’s Julius Caesar, and was likely written sometime around 1599, both Caesar’s army and those of Shakespeare’s own time, as today, contain military working dogs and canine mascots.
And the one thing that unites them all, is our desire to mark them as part of the unit.

Gefreiter Hund A German WWI era mascot dog complete with his own jacket with rank button, feldmutze with cockade and his very own Iron Cross Second Class. Photo via Flickr

1925. “Sgt. Jiggs.” The Marine Corps mascot in Washington, D.C., with an actual Marine. National Photo Company Collection glass negative

The official mascot of the United States Marine Corps, English bulldog Pfc. Chesty the XIV, sits for his official photo at Headquarters Marine Corps Combat Camera in the Pentagon, Arlington, Va, May 15, 2013 at Headquarters Marine Corps Combat Camera, Pentagon, Washington, D.C. Chesty the XIV will officially take over as the mascot when his predecessor, Sgt. Chesty the XIII, retires in the fall of 2013. (U.S. Marine Corps photo by Adrian R. Rowan HQMC Combat Camera/Released)
