Certain metabolites of some fungi (molds) are poisonous (toxic). Fungal poisons have been known for many years, but they were not considered a major factor in animal disease until fairly recently. Because the prefix myco- refers to fungi, these toxins are termed mycotoxins. Penicillin is derived from a fungus and could be termed a mycotoxin, in relation to bacteria which are sensitive to it.
Bagley, Clell, "Mycotoxins" (1997). All Current Publications. Paper 412.