A toxin is a poisonous substance produced by a living organism. A toxin can be a protein or small molecule.
In Foldit, many early puzzles featured protein toxins. A number of these toxic proteins appear as revisiting puzzles. There are two separate scorpion toxin puzzles, and also platypus venom, cytotoxin (from a cobra), cardiotoxin (from another cobra), and spider toxin in the revisiting series. A toxin from giant anemone is seen in another revisiting puzzle, although this protein can also act as a pheromone.
The toxins in these revisiting puzzles tend to be on the small side, with less than 60 segments (or residues). The platypus venom toxin has only 42 segments, which barely qualifies as a protein. Smaller chains of amino acids like the one found in playtpus venom are sometimes referred to as polypeptides.
Despite their small size, these frequently revisited toxins have a relatively large number of disulfide bridges, usually three or four.
Non-protein toxins have also been seen in Foldit.
Aflatoxin has been the subject of a series of Foldit puzzles. Aflatoxin is not a protein, but instead is a small molecule that is produced by Aspergillus flavus, a common mold. The goal of the alflatoxin puzzles in Foldit has been to design a protein capable of binding to the aflatoxin molecule.
See toxin on wikipedia for more on toxins.