Of course you can come up with your own stuff! Here's one that is medically based - Root of the Autumn Crocus. The autumn crocus is a flowering plant that has been used and abused throughout history. Medically, it is where the drug Colchicine comes from, and is one of the mainstays of treating gout. The compound has a narrow therapeutic window (the doses needed for treatment and toxic doses are close). When ingested, the compound causes symptoms of gastroenteritis (namely diarrhea).
Ancient Greek slaves would ingest the root of this plant to get off work, because it causes profuse diarrhea.
Now for the toxic effects:
The mechanism of action of colchicine is that it inhibits microtubule formation. . .why am I even talking medical right now? All you have to know is that the diarrhea coming from this plant can cause severe dehydration. At higher doses, it causes numbness, weakness, tingling of the extremities, and eventually ascending paralysis. Death results from either respiratory failure or multi-organ failure.
The therapeutic dose is 0.6 to 1.2mg daily, and the toxic dose is 7.2mg (that's when the deadly things happen).
How does this translate into your game?
When I read George R.R. Martin's books, I thought that poor Lord Doran Martell with his crippling gout could have used this, but his maester freaking sucks. In your setting, the old lady who runs the herbalist's store would probably have dried roots of this plant. Grind it up and put it in your favorite bad guy's food/drink, and you've got someone occupying the local outhouse for the better part of the day. I would personally make the Constitution Save DC15 for that effect.
If you want to be really mean, you can say that if the creature fails the save by 5 or more, they take 7d6 poison damage after 24 hours.