In D&D, the action economy makes sure that big boss monsters aren't actually boss monsters. This can be good or bad: if the boss comes after a lot of traps and minions, then the game poses a challenge: how to bypass the traps and minions without losing resources. The final fight will be a let down, however, if you're approaching this with a video game mindset. Two rounds, three rounds, who cares. It goes down. The alternative is to give these boss monsters wipe-out attacks: mind blast, dragon breath, or spells like cloud kill, fireball. Area effects which are close to save or die effects. That is, magic users, dragons, mindflayers. These monsters are basically total party kill challenges. Can you defend or preempt their wipe-out attack in order to make this a trivial fight, or will you all die? Challenges are good. This is good. But it collides with the video game mindset.
Let me now come to the boss monsters with many attacks, like squids and octopuses. What is good about squids and octopuses is that they have many attacks. If they have 6HD, 8 attacks, and AC 7 then you could also think of them as a bunch of low level minions attacking. The only difference is that they have pooled their hit dice and that they are better at hitting you. They are only a real challenge if the number of arms is significantly higher than the number of people in your party. If your party is four people, then a giant cephalopod is dangerous. If your party is twenty people, then you need five of them – or they're still no good. That is the first challenge.
The second challenge is fighting on the ocean. Giant squid and octopuses can attack and damage a ship directly instead of fighting sailors, but consider what this leads to: player characters have nowhere to run, have nobody to parlay with, and if they lose the fight, they will all drown in their armour. You need to think of ways to make this part of a good adventure. Is your party up for an underwater adventure? Is it plausible to assume that characters will survive a shipwreck? I'm not so sure. That is why I personally like to use squids and octopuses in underwater adventures: the party is already underwater.
This is probably my most cephalopodeous adventure: https://alexschroeder.ch/pics/16117940957_2a1cb1ed31_c.jpg
It's from this blog post where I discuss the question “If you use monster books, or even monsters from blog posts, what does your workflow look like?”

Anyway, all of these thoughts because of a new entry in my "boring monster manual":

What are your thoughts on using multi-attack monsters in D&D? Also, feel free to go back to the old Carcass Creeper or Carrion Crawler entry:
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