Arsenic is a chemical element with symbol As and atomic number 33. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. Arsenic is a metalloid. It can exist in various allotropes, although only the gray form has important use in industry.
Wikipedia describes arsenic as a nutrient thought to be important for mammalian health. This is likely to be a consequence of the chemical similarity of As and P. Sources of phosphorous will have traces of As in them, which means all organisms take in arsenic. Arsenic's toxicity comes from its ability to bind with thiols (-SH groups), not from its ability to mimic P.