If you look at Seismitoad's Pokedex Entry, it mentions 'paralyzing liquid' - i.e poison. Seis also can learn a number of Poison type moves, all of them resembling liquid (Acid, Sludge Wave etc). This is for the same reasons - shooting paralysing liquid. This liquid is obviously some kind of toxin. Palp can learn Poison type moves too, but none via level up. Toxin is a lesser factor with Palp, which is supported by there being a lack of actual reference to poison.