The octopus has the toxin. The chemistry, as far as the research shows, is done by guests. Endosymbiotic bacteria living in the posterior salivary glands produce the tetrodotoxin. The glands sit in the body cavity and have the structure of a secreting gland, a tubuloacinar exocrine gland, which is a precise way of saying they are built to make a product and send it out. The product leaves with the bite.
This is a pattern with company. Several animals that carry tetrodotoxin seem to get it from bacteria or from a diet that concentrates it, rather than from a private recipe in their own cells. The octopus version is tidy because the bacteria are housed where a venom gland should be, and the animal has sodium channels of its own that resist the toxin. It can keep a lethal neighbor in its mouth parts without switching its nerves off.
Tetrodotoxin is not confined to the gland. It has been found in many other tissues, including organs that have nothing obvious to do with biting. Mothers pass the toxin on to their eggs, and possibly the bacteria with it, so the young hatch already supplied. The animal is saturated with a defense it can survive, which is a strange kind of inheritance: a chemistry set installed before birth, aimed outward.
The partnership is the interesting natural history. A small mollusk, a bacterium, and a molecule famous from a fish are the same story told in a tide pool. It is also the reason homemade ideas about “milking” an animal or keeping one as a curiosity are worse than rude. You would be breaking up a partnership you do not know how to keep, in order to hold a poison you do not know how to undo.