I don't think you actually need to resort to magic, unless of course you absolutely want to. Of course you can add appropriate magic to any of these if desired (and allowed for by the rules of magic in your world). As you are asking about a magical creature, I suspect that magic is not limited to humans in your world; as such, it shouldn't strain suspension of disbelief too much adding magic to something like one of these, or you can base your creature off of one of these.
You say that you want something dog-like, in the size range of a poodle to a mastiff. Well, there are plenty of dogs that might fit the bill, particularly if actively trained for the work in question. Livestock guardian dogs provide a nice starting point, and the Wikipedia article has a pretty long, but likely not exhaustive, list of breeds.
Allow me to introduce you to the Anatolian Shepherd Dog:
You could also consider something like the Kuvasz:
Or try the Caucasian shepherd dog (kavkazkaia ovtcharka):
or the Great Pyrenees:
or any of several other breeds, many of which are bred purely for function and/or are recognized as purebred only by national kennel clubs, not internationally.
Livestock guardian dogs (LGDs) have been bred, often for thousands of years, to protect livestock against predators such as bears, wolves, and large cats. While much of the work they do is simply about being there (not many large cats are going to go teeth to teeth against a large, confident dog that stands its ground just for the possibility of a meal; there are easier prey), a hallmark of a well-bred LGD is that it also has the capacity to physically engage the predator if needed. As a consequence of this type of work, broadly speaking, LGDs tend to be bred to be wary of strangers, large but not extreme in size, physically strong, but also agile and not overbuilt, all of which are characteristics that you might want in an animal to be used for the purposes you describe.
Add a bit of directed breeding for willingness to follow commands from the handler (most LGDs are bred to work relatively independently, not like the dogs you see commonly which have been bred to work in close cooperation with the handler), as well as some specific training for how to bring down a human (read up on Schutzhund training, and look for videos on Youtube; there are plenty of videos showing the capabilities of "lesser" dogs, so you could just scale up a bit based on the breeding purpose of these dogs and the methods legally available to the guards), and you have a pretty formidable four-legged helper for the guards. By giving each guard party anywhere between two and a pack of these dogs (the number would probably depend mostly on how many guards there are in the party and how large groups people tend to form, the two of which are likely related), the dogs' mere presence (and the peoples' knowledge of what the dogs are capable of) should be enough to keep most people calm already, and the dogs certainly should be able to help the guards break up a brawl if needed.
For extra effect, if your society allows that, consider doing regular shows where a few people who have been convicted of serious crimes are given a close-range weapon (such as a knife or dagger) and a few of these dogs are pitted against them. Think something similar to the old Roman games where humans were pitted against wild animals. If the humans survive the encounter, they are free to go. There could even be a voluntary aspect to these; basically, you either take your chances against dogs and might end up dead or might end up free; or you are sentenced to life in prison. Now tip the scale ever so slightly in favor of the dogs -- you might send in one or two dogs more than you think you need, have the dogs commonly wear protective vests, spiked collars (the real stuff, not those just meant to look good), or whatever you prefer. Allowing or even inviting the people of your world to watch would give the -- hopefully large -- proportion of the people in your world who aren't likely to become major troublemakers an idea of what the dogs can do even to someone who is prepared and knows what to expect. There's nothing like a little psychology in the morning to keep people in line!