This is not a CR 1/4 creature. I just ran the Well Encounter at the Haunted House from Ghosts of Saltmarsh. I had four level 1 players and they got destroyed by two Giant Poisonous Snakes. There was not even a chance for them at all. On a hit the snakes do, at a minimum, 11 damage; At most 16; 22 on a crit! And what about the +6 to hit? Is this a joke? Two of the PCs were one-shot right off the bat. The next turn saw another go down and then the last player was running around with their head cut off. I wish they would actually balance the encounters or balance the monsters for the player character level they recommend. Am I supposed to be running every campaign with six PCs? I thought it was balanced for four… FFS. 

The robe of serpent but you can extract poison only if the giant poisonous snake  is dead.and if the snake die it disapear so you couldn't extract from this summoning snake. Perhaps, they can be a way that you can extract venom from a friendly our willing snake when alive like in real life .

How big is it

Quote from danielmaynard228 >>

How big is it

 It's sized medium, they could have easily made this a small creature, gnomes and halfling are size small, regular snakes are tiny. This has as much health as a wolf. Its bite without the poison deals as much damage as a decent hit with a weapon (from dexterity, I guess it goes for the neck?).
It would not make sense for it to be thinner than a thigh or big bicep. It should probably be long and very curled up in its space, but a shorter but still thick snake is also possible. Honestly WHY this is CR 1/4 instead of 1/2 is a mystery, or why it doesn't just deal less poison damage. This is a snake that's still most likely going to win a fight with a guard, even if that guard is completely immune to poison.

This is totally a CR 1/4 creature.... 
If you're a dwarf...

Quote from Sl4pch0p >>

On a hit the snakes do, at a minimum, 11 damage; At most 16; 22 on a crit!

 What are you using to calculate this? They do 1d4+4 piercing, which is a minimum of 5. The max is 8. Both in unison can potentially double that to 16, and 32 assuming two separate enemies both roll natural 20s AND the max on their damage dice.

They're still powerful but depending on your party's makeup you could easily take them down, since they're glass cannons. They should have easily been able to take at least one of them down within a turn, and the other should have followed. It seems like your players were all running around with their heads cut off.

Quote from Paragon_of_Life >>

Quote from Sl4pch0p >>

On a hit the snakes do, at a minimum, 11 damage; At most 16; 22 on a crit!

 What are you using to calculate this? They do 1d4+4 piercing, which is a minimum of 5. The max is 8. Both in unison can potentially double that to 16, and 32 assuming two separate enemies both roll natural 20s AND the max on their damage dice.

They're still powerful but depending on your party's makeup you could easily take them down, since they're glass cannons. They should have easily been able to take at least one of them down within a turn, and the other should have followed. It seems like your players were all running around with their heads cut off.

 You are ignoring the 3d6 poison damage. Minimum damage if they make the save vs poison is 6. Max damage if they fail the save vs poison is 26! 

Average damage roll is 11 (which assumes you make the save), or 16.

No one going to say anything about the fact that you can make a snake prone lol

Well this one is poisonous so just don't eat it.

Some snakes in the wild are indeed poisonous, such as the tiger keelback. Interestingly enough, they dont even make this poison either, they just kinda recycle it from the toads they eat and use it for themselves. Really interesting stuff, Nature is wild!

I feel like this is supposed to be a big venomous snake. Calling it poisonous is a tad confusing as it is either A. Mostly incorrect, B. Intended to be both Poisonous and Venomous, or C. Supposed to be Poisonous and it somehow is just dealing extra damage with poison despite that not being how most poisons works.

Venom is a very specific type of poison. It works via Injection. Most kinds of poisons work via Ingestion.

Toxic or Toxin is the word used for broader uses of some form of chemical agent, biological or otherwise, that causes harm.

Giant Toxic Snake would cover all the bases with no confusion as something that is Toxic is likely to harm you however those toxins get into your body. Rather than the specific natures of the other two. And if not, Giant Venomous Snake is probably much closer to what they were actually going for. 

It's a venomous snake. If you look into it the "poison" you get from it is a poison that can only take affect if introduced through wounds, called a injury type poison. The name of the poison is even Snake Venom.

Serpent venom (Injury): This poison must be harvested from a dead or Incapacitated Giant Poisonous Snake. A creature subjected to this poison must succeed on a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.

So yeah it's still a venomous snake.

Quote from Paragon_of_Life >>

They're still powerful but depending on your party's makeup you could easily take them down, since they're glass cannons. They should have easily been able to take at least one of them down within a turn, and the other should have followed. It seems like your players were all running around with their heads cut off.

 That encounter usually starts with falling into a well, forcing your party to kind of trickle in. With their decent AC and not too bad hp they can survive long enough to pick the party off one at a time. The only thing stopping the encounter from being too hard is that you can kill them at range then go save your friends... if they made their death saves.

not to be that person but there are plenty of snakes that cannot and will not kill you. For example, my pet snake George

Great for wild-shape if you have unarmoured defence (monk).

Are venomous snakes immune to poison?

i like big snake

I literally just did and it was the best time ever