This is not technically a poison, but it's a good example of something we'd consider barmy today. (And I had to start somewhere). It's also entertaining because the practitioners of quack medicine are mad keen cuppers. One of the most startlingly obvious features of quack medical practices is that - irrespective of the modality - they are very often something that has been discarded by medicine during its evolution. Only the really whacky ones aren't.
Cupping is an ancient approach, that much is true. Egyptians were cupping around 1000 BC and the practice may be older still (and have evolved more than once, in different places). The Hippocratic and Galenic traditions that physicians still adhered to at the turn of the 19th century encouraged cupping for a variety of ailments, and the practice only died out over the following hundred years.
There were two forms of cupping, which were often mixed together. The basic process was common to both: heated cups were placed over the skin, and as the air cooled inside them (and the pressure dropped) they sucked the skin outwards. That much is dry cupping. Wet cupping involves scarifying the skin before applying the cup, and was a form of bloodletting. The two forms were often mixed, dry cupping being applied first to form a blister, which was subsequently lanced and re-cupped to be bled.
How did it work? It almost certainly didn't do any therapeutic good whatsoever. The idea of bloodletting came from the ancients and was thought to restore the balance of humors in the body (they being blood, phlegm and yellow and black bile). Bloodletting in specific locations on a small scale was thought to draw inflammation away from nearby tissues. On a larger scale, patients would be bled from a large vein until they fainted. Since all conditions were thought to be due to an imbalance of the humors, this practice was quite widespread until the revolutions that occurred in medical practice during the 19th century. Untold unnecessary deaths resulted. On the other hand, more benign dry cupping probably arose from the notion of counter-irritants - the idea that a disease-induced irritation could be resolved by a deliberate one elsewhere.
Modern quacks promote dry cupping as an ancient and traditional Chinese healing system, similar to acupuncture. Western medicine employed it for thousands of years, and only discarded it (with a lot of other practices) when people started to pay careful attention to whether treatments actually worked or not.