$\ce{H2As(=O)OH}$ and its $\ce{As}$-hydrocarbyl derivatives, e.g. $\ce{Me2As(=O)OH}$, dimethylarsinic acid.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1320 [Terms] [Paper]