Here are some ideas for Pikmin enemies in Adventure mode that I had. I've listed each enemy, which games they appeared in, their characteristics, and how I feel they should behave/attack. Hope you like.
[COLLAPSE="Normal enemies"]
[COLLAPSE="Grub-Dogs"]
Red Bulborb
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Probably the most well-known enemy in the Pikmin series. If Pikmin enemies do appear, it would only make sense for the Red Bulborb to appear.
Behavior:
In the series, Bulborbs usually stay asleep until attacked. This behavior could easily be dropped, especially if they appeared in a nighttime area.
Attacks:
All of these are based on behavior in the Pikmin games
-Run and lunges at the player.
-Shakes itself if it is being attacked.
-Bites/tries to eat the player(maybe tosses the player up in the air?)
Orange Bulborb
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Debuting in Pikmin 2, the Orange Bulborbs behavior is very similar to the Red Bulborb.
Behavior:
The main distinction between Red and Orange Bulborbs is that Orange Bulborbs are easier to startle, meaning that they wake up easier/faster than other species. They aslo have mor health than other Bulborbs.
Attacks:
Same as Red Bulborb, once woken up.
Spotty Bulbear(and Dwarf Spotty Bulbear?)
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In Pikmin 1, The Spotty Bulbear was just a bigger, stronger version of the Red Bulborb. In Pikmin 2, however, it gained the distinction of being one of the few Grub-dog species that would be constantly awake when the Player encountered them. They would patrol a specific area, often being followed by Dwarf Bulbears.
Behavior:
As mentioned, it should pace a given area until it saw the player character, and then would begin to pursue the player.
Attacks: Same as the Bulborbs mentioned.
Fiery Bulblax
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Fiery Bulblaxes are somewhat rare enemies in Pikmin 2. They behaved similarly to other Grub-Dogs, aways being asleep when the player encountered them. The main difference between the Fiery Bulblax and other species, as you probably already figured out is that the Fiery Bulblax is always on fire.
Behavior:
Would behave identically to the Red Bulborb.
Attacks:
Most of it's attacks would be the ones already listed.
The fire: since the Fiery Bulblax is, y'know, ON FIRE, naturally the fire would damage the player. I'd suggest that the fire did small continuous damage to anything close to it, but without any knockback.
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Breadbug mimics
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These are not actually smaller Bulborbs, but are creatures who mimic the appearance of Bulborbs to help protect themselves.
Behavior:
They would wander around, usually close to a Mature Bulborb. If startled/ attacked, some might let out a cry that would wake any nearby sleeping Bulborbs.
Attacks:
-Rush the player and lunge at them.
-Shake, dmageing players close to themselves.
Wollywog/Yellow Wollywog
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Wollywogs(Wollyhops in some regions) are found mostly in aquatic areas. They behave mostly identically to each other: once prey is within a cartain distance, they hop up in the air and fall to crush/eat their prey.
Behavior/Attacks:
As I mentioned, they would wait for the player to be within a certain range, hop up, then fall to crush the player.
Fiery Blowhog
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Fiery /blowhogs appear in both Pikmin games. Their main attack is their ability to spray fire.
Behavior:
Would wander about a giver area until they noticed the player, then would attack.
Attacks:
-Primarily, they would spit fire at the player.
-If the player got too close/ attacked the Blowhog, it would buck, dealing high knockback.
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[COLLAPSE="Rarer/more difficult enemies"]
Puffy Blowhog
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Appearing in both games, the Puffy Blowhog is an airborn and somewhat difficult enemy to deal with.
Behavior:
Float around a specific area. If it noticed the player, it would move a certain distance from the player and ready its wind attack.
Attacks:
-Blow a powerful gust of wind at the player.
-Shake itself if the player were too close.
Burrowing Snagret
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A rare boss-like enemy in both games. Burrowing Snagrets remain hidden in the ground until approached. Its head is its only weak point.
Behavior:
Buried until approached. When the player is nearby, it emerges from the ground, attacks, and then burrows.
Attacks:
-Emerge from ground
-Peck at player.
Pileated Snagret
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A rarer, stronger relative of the Burrowing Snagret found only in Pikmin 2. Unlike the Burrowing Snagret, it can actually move once it emerges from the ground and could kill pikmin when it dove back underground. Like the Burrowing Snagret, it can only be damaged by its head.
Behavior:
Buried until approached. When the player is nearby, it emerges from the ground, attacks, and then burrows.
Attacks:
-Emerge from ground
-Peck at player.
-Dive and burrow into the ground
Armored Cannon Beetle
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A boss-like enemy appearing only in the first Pikmin. It can only be defeated by attacking the red area on its back that it keeps covered.
Behavior:
Patrols around a small area. When it notices the player, it faces them and begins to suck in air through the white hole near the top of its shell, readying its boulder attack. Note that in the original Pikmin, it had to suck in and “choke” on a Pikmin to lift its “wings” and expose its vulnerable backside.
Attacks:
-Shoots large boulder at player.
Emperor Bulbalx
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The final boss of the first Pikmin, it was downgraded in size and became more common in Pikmin 2.
Behavior:
Would remain buried in the ground, only its eye stalks visible. If the player walked over the eyes, it would emerge from the ground.
Attacks:
-Bursting from the ground(1-time only), deals some knockback but no damage.
-Shooting out its tongue.
-Shaking itself if the player is too close.
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Boss time!
[COLLAPSE="Boss"]
Titan Dweevil
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Note that here, the Dweevil is shown without any of it's weapons/armor.
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The final boss of Pikmin 2. Attacks with a variety of weapons(which are actually treasures). In Pimkin 2, ALL of its weapons had to be damaged/knocked loose before the main body of the Dweevil could be damaged.
Behavior:
Walks around over the player. Attacks with its weapons at constant intervals.
Attacks:
Flare Cannon: Sprays fire on one side of the Dweevil.
Shock Therapist: Shoots out electrical nodes, then some chain lightning
Monster pump: shoot balls of water into the air, which wall and damage the player.
Comedy Bomb: Sprays 3 rotating streams of poison gas. The gas could have various effects, like making the player fall asleep or giving a Lip's Stick flower.
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tbrawl:, :snakebrawl:, :sonicbrawl:, and maybe :mariobrawl:, :luigibrawl:, 
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