These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anticlimax Boss: Le Rauxe is the one that imprisons Claw at the very start of the game, forcing him to get through the whole prison and battlements. You can beat him to a bloody pulp in less than 10 seconds. For a guy who managed to sink Claw's ship and imprison him, he's surprisingly inept when it comes down to swordfighting.
Demonic Spiders: Tiger Guards from the last level can easily dodge all your sword attacks, require THREE of them to take down, and are immune to Pistol and Dynamite. Not to mention they can deal up to 20 damage (which is a fifth of your health) and tend to cooperate with certain other enemies.
Seagulls. Bloody birds, why would they be attacking an escaping pirate?
Mice who throw bombs and kinda hard to kill without getting hurt, especially if there are more of them at the same spot.
Goddamned Chameleons: The sole level they appear in is infested with them.
Peg Legs. They are very fragile (dying from crouch sword-normal sword combo while everything else requires two normal swords.
Crazy Hook requires five crouches to be put down), but if they stun-lock you in their two-shot combo, you die. No exceptions. They can also avoid your pistol shots, which makes them frustrating to take down if you want to jump on another platform.
Bear Sailors. Enjoy half of your health power gone in their hug. And they can repeat it just a second later!
Marrow (whose gun hurts like hell and there's also that parrot of his) and Red Tail (with his wide variety of attacks and unfriendly environment around) count as well. There's also Omar (but he's a Final Boss) and Catherine (but she's a Wake-Up Call Boss).
Omar. Even despite the fact he's Final Boss, he deserves a mention: People were known to lose all of their lives just to get past him.