It is unknown whether this was an official position on Terran Empire starship, or how long this position has existed. Archer could have just created it to secure his leadership. When Archer captured Captain Forrest, the commanding officer of Enterprise until Archer's mutiny, Forrest was only accompanied by a single MACO. It therefore seems likely that the position existed before the mutiny. It should also be noted that Spock having a personal guard of his own showed how almost any one in a high ranking position could have one.
I'll rewrite to remove speculation. --LauraCC (talk) 17:30, March 29, 2016 (UTC)
There are a couple of DS9 references to "personal guard" in "Crossover" and "Image in the Sand" (the latter is a non-mirror universe reference). Even if there weren't, the fact that mirror Spock had one is reason enough, as he wasn't the captain yet.
That said, is there anywhere in the scripts that the job is referred to as "captain's personal guard"? If not, swap the redirect with the official page and delete "captain's personal guard" --LauraCC (talk) 17:37, March 29, 2016 (UTC)