Alternative Bosses

Alternative Bosses are anomalies that can appear in seemingly every course, where a boss can be replaced with something else, or a new boss can appear where there is none.

Chain Chomp
Bob-Omb Battlefield's alternative boss is the Chain Chomp. The regular Chain Chomp can appear at the top of the mountain, where the post must be ground pounded 5 times, instead of the regular 3 times. The Chain Chomp usually acts the same, but could obtain new attacks and behaviors, though this is unconfirmed. When the post is ground pounded, it will jump off the mountain, destroy the gate blocking the star, and jump away, similar to the regular mission.

Queen Thwomp
Whomp's Fortress' alternative boss is Queen Thwomp. It can replace King Whomp, but can appear in seperate missions, replacing the tower at the top of the course. It is a pink Thwomp that is twice as large as the regular Thwomp. It attacks by moving above Mario and attempting to crush him, and creates a shockwave similar to final Bowser when it hits the ground. It must be defeated by being jumped on top off and ground pounded 3 times. It will explode into a star.

Unagi
Jolly Roger Bay's alternative boss is the Unagi. It is indicated the alternative boss is activated when a cutscene plays that zooms in on the Unagi. It attacks like the usual Unagi, and must be defeated by running into them with the Metal Cap 3 times (The Unagi will be knocked back each time so it isn't defeated instantly). When defeated, it will crash into the wall, fall down, and cough up a star.

Yeti
Cool, Cool Mountain's alternative boss is the Yeti. It can appear seemingly anywhere in the main course. It is a Bowser reskin with white fur covering most of it's body, with it's face being a pale skin color. It attacks by attempting to run into the player, and must be defeated by making it fall off the course 3 times (It jumps up after each fall, similar to Bowser). When defeated, it will not jump up, being replaced by a star.

Mad Piano
Big Boo's Haunt's alternative boss is the Mad Piano. It functions as the brown Mad Piano enemy discoloration, taking 3 ground pounds instead of 1. It will explode into a star.

Flaming Face
Lethal Lava Land's alternative boss is the Flaming Face. It takes the appearance of the flame with a face on the painting, and can appear instead of either Big Bully. It attacks by shooting fireballs and, oddly, homing Bullet Bills at the player. The Flaming Face is defeated by making it get hit by a Bullet Bill 3 times, which will cause it to evaporate, and a star will fall from the sky.

Power Pokey
Shifting Sand Land's alternative boss is the Power Pokey. It is a regular Pokey with 10 body segments, and is defeated like a regular Pokey. It can appear in place of a regular Pokey. When destroyed, a star will fly into the pyramid, which the player must enter to find the star. The star itself can appear seemingly anywhere in the pyramid.

Ghastly Snufit
Hazy Maze Cave's alternative boss is the Ghastly Snufit. It is a large version of a Snufit, but with a purple body instead of a regular Snufit's red. It can appear in the Hazy Maze in place of a regular Snufit. It attacks the same as a regular Snufit, but Mario can punch the bullets to send it back at the Ghastly Snufit, which must be done 3 times to beat it. When defeated, it will explode into a star.

Unagi (Rematch)
Dire, Dire Docks' alternative boss is the Unagi. It will appear out of a new tunnel with a bandage on it's head, and will only appear if fought in Jolly Roger Bay first. It is fought the same way, and the star will appear the same way.

Big Shocker
Wet-Dry World's alternative boss is the Big Shocker. It is a large Amp that can appear in the village section of Wet-Dry World. It attacks by shooting balls of electricity at Mario. Several flamethrowers are scattered around the Big Shocker, and must be punched to be activated and damage the boss, and then it will break apart. 3 different flamethrowers must be activated to defeat the Big Shocker. When defeated, it will start shrinking until it is the size of a regular Amp, and then explode into a star.

Rocky Wrench
Tall, Tall Mountain's alternative boss is the Rocky Wrench. It will appear at the top of the mountain and take the appearance of a regular Rocky Wrench from other Mario games, and will attack the same way, by throwing wrenches at Mario. It has to be defeated by being punched in the back to knock the platform off of it's head and then ground pounded 3 times (each time, it will retreat to another hole in the ground). When defeated, it will fall into the hole it is in and a star will pop out.

Snowy Spindrift
Snowman's Land's alternative boss is the Snowy Spindrift. It is a regular Spindrift with blue petals. It attacks by running at Mario, and jumping up and spinning it's flower to blow snow and wind at Mario, attempting to knock him into a freezing pond. It is defeated by being tricked into running into a freezing pond 3 times (It will jump out each time). When it falls into a freezing pond a third time, it will sink and a star will come from the pond.

Petey Piranha
Tiny-Huge Island's alternative boss is Petey Piranha. It will take a similar appearance to it's Super Mario Sunshine appearance. It will appear on the top of the huge island, and shoot fireballs at Mario as he climbs up. It attacks by attempting to jump on Mario and shooting fireballs at him. It is defeated by punching it's belly button 3 times. It will fall down the island into the ocean, and a star will pop out of the ocean where it fell.

Clockwork Mega-Machine
Tick Tock Clock's alternative boss is the Clockwork Mega-Machine. It's design varies, but usually takes the appearance of a large Metal Mario. It attacks by attempting to crush Mario. There are several platforms around it, which Mario must climb, avoiding being punched, to jump on top of Metal Mario and hit a red button on the top of it's head, which will cause it to explode, and it's cap fall to the ground. A star will pop out of the button afterwards.

Sky-High Guy
Rainbow Ride's alternative boss is the Sky-High Guy. It is a large Fly Guy with a blue body instead of red. It usually appears at the edge of the course. When Mario walks near it, it will turn to face downwards and spin it's propeller to send Mario flying off of the course. A cannon will appear on the airship, which must be punched by Mario to hit the Sky-High Guy, which will fall down and leave a star behind.

Ent
Fox Forest's alternative boss is the Ent. It can replace the final mission and instead be "Ent's Offensive". It will appear in the center of the course, and will take the appearance of a toad sized tree with red eyes and 4 spider-like legs. It attacks by throwing apples out of it's leaves, and has to be jumped on 3 times to be defeated. It will fall down, and a star will pop out of it's leaves.

Metal Bowser
Eternal Fort's alternative boss is Metal Bowser. It is a boss similar to final Bowser, but using a Metal Cap. His tail cannot be grabbed, and can throw Amps at Mario. When an Amp is thrown, Mario must punch it when it's electricity disappears for a short timespan to damage Metal Bowser. When defeated, Metal Bowser drops a key that has varied uses.

Theorizing
It is unknown why courses already with bosses have alternative bosses, though the Personalization AI could do it to change the difficulty for players. The bosses that appear in courses without regular bosses imply that every course was planned to have it's own boss fight.

Reportings
''Everything in this section should be taken with a grain of salt. These are all unverified claims.''

Report 1
"I was playing Mario 64 on an old N64 I found in my attic, for nostalgia. Everything was normal until Jolly Roger Bay. I finished all 6 missions but then when I went to get the 100 coins star I saw a 7th mission, "Unagi's Hunt". I selected it and there was this cutscene of the Unagi exiting it's tunnel and clacking it's jaws. I grabbed the Metal Cap and ran down towards it. It started swimming straight towards me, so I ran around it and jumped into it. It recoiled backwards and I realized I did damage, and the Metal Cap wasn't disappearing. I hit the Unagi 2 more times, and it recoiled into a wall, falling to the ground, then coughed up a star. I moved on, thinking it was a special mission I somehow unlocked. When I went into Dire, Dire Docks, I did all the missions again, went to get the 100 coin star, and "Unagi's Return". It was the same as before, except the Unagi had a bandage on it's head. I grabbed a Metal Cap nearby I didn't remember was there and fought it. I defeated it, and the same defeat animation played. Nothing else out of the ordinary happened, and I only recently learned this wasn't a normal occurrence. I don't know why I found the Unagi boss fights, but they were, honestly, pretty fun."