Ideas for a Legendary Poison Fakemon

Pretty much what the title says. I'm working on a Pokemon fanfic with plenty of fake Pokemon, and I decided to make a Poison-type legendary. I'm struggling with the design, and figured, hey, this would be the place to ask.
I've already got its role in lore mostly laid out: the ancient monster laid waste to cities and was the source of all disease, so the people of the world sealed it away to prevent it from wiping out all existence.
I'm currently entertaining two design possibilities: the first is simple, a massive lumbering pile of sludge. It's classic, but it would be tricky to distinguish it from Muk, and it just doesn't feel legendary. The second is to take the generic anime dragon that's so popular these days... and melt it. Slimy flesh dripping onto the ground, mouth hanging open, jaws connected by strings of sludge... This idea's solid, but I don't feel like an anime gives the Poison type justice.
If anyone can think of ways to expand on either of these ideas, or has a new one altogether, let me know. Ideally, I'd like a pure Poison-type, but if anyone has a good dual-type concept it's certainly not out of the question.

A large purple and blue scorpion pokemon. There's not enough scorpions. Never enough scorpions.

Ooh, I hadn't even thought of that! I still feel like some sludge is needed... maybe have sludge leaking out of seams in the exoskeleton?

An enormous skeletal metal cockroach, basically a personified factory, perpetually leaking fumes and sludge.

Would some sort of Nuckelavee Pokemon work? Maybe some sludge-skin falling off, or oozing toxic fumes from parts of its body. You could have it be sealed under a giant lake of purified water.

The rest of the suggestion I can take or leave (I'm trying to make the monster designs something that might actually be in an official Pokemon game, so nucklelavees are out), but this. This is pure gold.

Nuckelavee may work if he is heavily stylized. There are a lot of ways to visually interpret a mythological creature like him.

But the complete lack of skin doesn't exactly lend itself to a children's RPG.

You don't have detail the muscles or veins or anything.

Think Meat Boy.

It should be made of the souls of every single Pokeblock poop that has ever happened.

Pretty much this. We've had some hideous things in Pokemon anywho, including a sperm cell in the Homonculus family in Gen V.

As for my idea, perhaps some sort of massive mushroom colony, spewing miasma? The thing would be found under a swamp. The lake of pure water is a great idea too. I'd also throw in a rivalry with Suicune, since it has the ability to purify water.

May I suggests a zombie of some sort.

So at this point, we've got a Steel/Poison metal plated Nucklelavee (i'm thinking just give it the basic layout), with smoke rising towards the sky and sludge leaking out everywhere, at the bottom of a purified lake.

If you gave it steel-poison i think it'd become much more of a 'steel' legendary by that point.

things were so much simpler when it was a melting dragon