No such thing as a lone minion. If you come across minions, it will be, by definition, in a group of four or more.
Furthermore, your example is explicitly ruled out by the rules we've seen so far:
Here's the brain bender: The minion template is an encounter mechanic, not a monster mechanic. Examples of monsters with this mechanic applied are given in the MM so the DM can quickly build encounters using pre-fabricated minions, not because minions can be encountered solo 'in the wild'.
Minions drop in one hit because having a dozen or more guards who fall quickly to the heroes' attacks is a common trope of adventure fiction. The minion rules allow an encounter to include that trope, but outside of that encounter those rules break down. Minion rules are only meant to be used within the context of a larger, level-appropriate encounter.
Case in point: Humans can be minions. That doesn't mean that there are human lackeys walking around in the gameworld who live in fear of being attacked by a small dog. And just because they signed up to be a lackey doesn't mean they've suddenly dropped to one hit point, or even that they temporarily acquire the structural strength of a wet noodle whenever they're 'on the clock'.
So, if you come across such an NPC by themselves, then it would probably be a normal creature because the DM hasn't placed it into an encounter using the minion mechanic.