Okay, so first things first we have to try to ignore negative evidence as much as possible. Meaning that just because we've never seen something, that doesn't mean it doesn't exist: it's just not considered interesting enough to appear in the show yet. The show uses this a lot, see Shining Armour himself and the fact he's not mentioned until suddenly Twilight has a brother and he's getting married. So just because we haven't seen the guard stationed in Ponyville and other areas, doesn't mean they're not there. We just haven't seen them for some reason. Also just because we've not seen the guards act in a competent fashion with regards to threats against Celestia and Equestria doesn't mean they're incompetent at their jobs, it's just means the stories we've had so far did not focus on the guards being competent.

The Guard gives the impression of being a professional group, rather than a militia or mercenaries, and are combining aspects of police and armed forces. This is surprisingly rare historically, despite how common it is in fiction. Standing armies are almost never used for regular city watch duties until you wander back to the Imperial Roman era, or forward into the World War periods and onwards. For that matter, standing armies themselves aren't as common as people think, especially in feudal systems where the whole arrangement is usually based on militias rather than professional soldiers. So to make this easier on everyone, I'm going to restrict this to modern organizations that seem to parallel the Guards.

From an American point of view, the Guard appear to have similar roles to the American National Guard, with aspects of the Secret Service (guarding the Princess), and the Marines (guarding embassies/palaces/etc.). And then the Wonderbolts are obviously modelled after the Blue Angels acrobatics team, whose primary purpose is actually recruitment drives.

However, take things north of the border, and there is an organization that more closely maps to the Guard in its entirety: The Mounties.

I'm not kidding. The Royal Canadian Mounted Police is more like the MLP:FiM Guard than anyone else I’ve ever run into. Bear with me here.

The RCMP is technically a federal police force, but also acts as a provincial (read State for those in countries with states rather than provinces), and a municipal police force. In most of Canada it is the only police force. They provide protection services for the Canadian leadership the same way the Secret Service does in the 'States, and they co-ordinate the Canadian emergency services, border patrols and coast guard the same way as the American Homeland Security does. At one point they also had all the duties that the American CIA had, until 1987 when it was re-organized. They also have an 'acrobatics' troupe: The Musical Ride (awful name, I know), that combines drill (synchronized riding) and vaulting (gymnastics and trick riding). Plus they used to have the Canadian version of the Scouts as a cadet group, in a manner awfully similar to the 'Junior Speedsters' mentioned by Rainbow Dash.

But the bit that cinches it for me, and the part that nobody ever seems to remember, is that in 1921 the RCMP was granted the status of a full-blown military unit, and from there stepped into a similar role as the American National Guard . During the World Wars, the RCMP officially fulfilled the Military Police role for all Canadian Armed Forces. All this seems to have been forgotten when Canada officially stopped being a British Colony in 1982, but if you dig into the books; technically the RCMP is still a regiment of Dragoons (combined Light Cavalry & Mounted Infantry unit) as part of the Canadian Armed Forces, and is also still a division of the British Royal Horse Guards.

That's right; the Mounties are part of an Equestrian Guard.