So you're crawling through that dungeon, halfway to its boss. An enemy appears, but as you ready your weapons to fight, you discover this one's different. You're going to need a lot more to kill this thing.
A Boss in Mook Clothing describes those enemy encounters which could well be proper boss fights due to their difficulty. Generally they have much higher hit points and/or defenses than regular enemies, deal a large amount of damage with each attack and have no explicit or obvious weakness which can be used to take them out quickly. They just require some strategy and hard work, usually without the same payouts of a regular boss.
Some other common traits of these enemies can include the ability to cast multiple status inducing moves and having a much wider variety of attacks compared to the average enemy.