This watercolor is based on a faience hippo figurine from the 13th Dynasty of Ancient Egypt.
Egyptian faience was made by mixing crushed quartz or sand with copper salts and a binder like lime or resin. When a figurine was fired at a low temperature, the copper salts turned into a glaze, creating a green or blue hue. It was one of the earliest forms of glass making.
I saw this hippo at the St Louis Art museum on a trip we took March 2009.