Alternative Bosses

Alternative Bosses are anomalous bosses that can be found in seemingly any level in Super Mario 64. They will usually replace the bosses in their course unless there is no boss in the course to replace, though regular bosses not being replaced is not uncommon. It is believed that Alternative Bosses are created in order to change the difficulty according to the player's playstyle.

Examples of Alternative Bosses
Below is a list of Alternative Bosses. Note that these are the most common examples, and are not the only possible examples.

Chain Chomp
Bob-Omb Battlefield's alternative boss is the Chain Chomp, being one of the few examples of Alternative Bosses that are already enemies in the game. The Chain Chomp can appear on top of the mountain, replacing Big Bob-omb, instead of their regular spot. They otherwise function as normal, and will break the gate when the post is ground pounded 3 times.

Queen Thwomp
Whomp's Fortress' alternative boss is Queen Thwomp. It can replace King Whomp, but can appear in separate 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. Queen Thwomp can be defeated by punching it when it falls to the ground 3 times. When defeated, it will drop 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, and is also the same color as the regular Mad Piano. 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. Whenever it shoots a fireball, it gets slightly smaller. It is defeated when it shoots too many fireballs and becomes so small it is barely visible. When defeated, a star will spawn.

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 appear.

Snowman's Land's alternative boss is the Snowy Spindrift. It is a regular Spindrift with blue petals instead of pink. It attacks by running at Mario, doing damage upon contact. It has to be jumped on 3 times to be defeated, though the player does the spin-jump each time they jump on the Snowy Spindrift. Upon defeat, a star will spawn.

Petey Piranha
Tiny-Huge Island's alternative boss is Petey Piranha. It will appear as a large Piranha Plant at the top of the huge part of the level, not to be confused with a large variant otherwise regularly found on the huge part. When the player reaches Petey Piranha, they simply need to punch it while it is sleeping, making it disappear and turn into a star.

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, indicating 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."