Examples:

The Goomboss from Super Mario 64 DS is the largest* Goomba, and has the ability to create more goombas from nothing.

*The Mega Goomba from New Super Mario Bros is bigger, but lacks the ability to spawn goombas, and is likely only able to grow to that size due to a mega mushroom, which debuted in the same game.

King Kaliente from Super Mario Galaxy is the ruler of the Octoomba species, for obvious reasons.

King Bob-omb, king of the Bob-ombs and other military ordnance type enemies, such as Bullet Bills, Torpedo Teds, etc.

King Whomp is the ruler of the Whomps, Thwomps, and other large, stone enemies like Tox Boxes. What gives the Whomps an edge over the other subspecies is mobility; Thwomps can only move up and down or side to side, Tox Boxes can only move in a grid, but Whomps can walk around freely, restricted only by their size and stiffness.

Petey Piranha is NOT a king, because (at least in his first appearance in Super Mario Sunshine) he is not a living creature, but a creation made by the magic paintbrush. After that, he seems to basically be a wild 'animal'.

Donkey Kong and King K. Rool are the leaders of the Kongs and the Kremlings, respectively.

And of course, there's Bowser. His normal size is already pretty big compared to every other koopa*, but he can also grow to kaiju size in a variety of ways (adrenaline, rage, magic, etc.) He also dabbles in dark magic, being able to turn people into brick blocks and teleport, among other things. *There is the Mega Koopa from Super Paper Mario, but again, that's only because of a power up, and Bowser is capable of growing much larger than that. Being the most powerful among the monarchs gives him authority over most kingdoms.

And then there's King Boo. There are Boos that are bigger than him (Boohemoth, Boolossus, etc.), but none of them come close to King Boo's proficiency in magic, singlehandedly performing tasks that would take tens, if not hundreds of boos to accomplish, like materializing entire mansions. In my opinion, it seems like Boos aren't particularly loyal to Bowser, only fighting Mario because they enjoy it.

Peach is the ruler of the Mushroom Kingdom and its peaceful denizens (Humans, Toads, etc.) because of her own combat and magical abilities.

And yes, I am currently unemployed. How could you tell?

Most of us like myself were first introduced through Super Mario Bros. 3 for the NES/Famicon, but those of you who were introduced to this game through the Super Mario All-Stars version from the SNES or the Game Boy Advance version through Super Mario Advance 4 can also participate.