So, you might be asking yourself... what's a special Mii?
Special Miis are Miis that fall under two categories:
The first category are Miis that aren't technically Miis, but are instead usually things like premade 3D models, instead of being read from a Mii data file like normal Miis. These are called Fake Miis. These Miis have a pink banner above their icon on the Mii Library.
The second category are Miis that usually have golden pants. They're almost always distributed in ways that are different than others. They usually don't appear in games, and many times are done via special one-time events or QR codes. These are called Golden Miis (even if they don't necessarily have golden pants). These Miis have a golden banner above their icon on the Mii Library.
Now, I'm using words like "usually" and "almost always" because there are a few exceptions to each of these rules. But most of the time, this is pretty much the best way to describe them. As for Golden Miis, how I obtained each of those Miis will be explained in their page, and since usually how I obtained the Miis and how the way the Miis were originally meant to be obtained are different, I'll also explain the original methods of obtaining each Mii.