I'm thinking of playing Pikmin, but I'm curious as to what boss battles are in the game. Can you put together a list of Pikmin enemies that people consider bosses? Also, can I have an opening sentence or two going through what defines a boss in the game?
I can do that for you! Bosses are defined in two ways. Some bosses are labeled in the game's files as bosses, and others aren't labeled bosses but are called bosses by people who play the game. Here is a list of bosses in Pikmin:

1. Goolix - The Goolix is a Pikmin boss that looks like a blue amoeba. It fights by encapsulating Pikmin within itself, which will drown them. It is impossible to fight if you don't have blue Pikmin, but blue Pikmin can fight it without being injured. You can only find the Goolix after day 9 on odd-numbered days in the impact site,
2. Burrowing Snagret - Snagrets are creatures that pop out of the ground and look like a mix between a snake and a bird. When they fight, they unburrow and eat several Pikmin and then burrow back down for safety. In the Forest of Hope, there are 3 Snagrets in the same location. The best way to fight a Snagret is to attack as they try to emerge from the ground. Then before it has a chance to shake them off, whistle them back.
3. Beady Long Legs - Beady Long Legs - Arachnophobes beware, the Beady Long Legs is a massive spider. It stomps around, and Pikmin die as it steps on them. It also drops from the ceiling abruptly when you find it, which can immediately crush Pikmin. It helps to fight it with a smaller team of Pikmin, as large teams can more easily be crushed. It also helps to have Yellow Pikmin, as they can be thrown higher than others. This boss resides in the Forest Navel.
4. Armored Cannon Beetle - The Armored Cannon Beetle is a boss that isn't labeled as a boss. This beetle has armor surrounding its body. It also has a blowhole that sucks in before it shoots out a boulder. The ball that rolls can crush several Pikmin and only stops when it hits a wall or other enemy. To fight it, you need to clog its blowhole, which will then make it expose its abdomen which can be attacked. Its blowhole can be clogged by Pikmin. You can find the armored cannon beetle in The Forest of Hope.
5. Emperor Bulblax - This is the final boss of Pikmin. It looks like it's covered in sludge with moss growing out of it. The Emperor Bulblax waits in the ground for its prey and pops up as you pass by. To defeat this enemy, you need to use Yellow Pikmin to feed it bomb rocks. It fights by eating, and when it sticks out its tongue, you can throw the Pikmin with bomb rocks at it. This boss can be found in the area called The Final Trial.