A yi is a kind of ancient Chinese ritual bronzes in the shape of a half gourd with a handle (often in the shape of a dragon), usually supported by four legs. It is believed that it was used to contain water for washing hands before rituals like sacrifices.
References http://arts.cultural-china.com/en/30Arts2067.html (unsafe URL) http://arts.cultural-china.com/en/30Arts2015.html (unsafe URL)
External links The great bronze age of China: an exhibition from the People's Republic of China, an exhibition catalog from The Metropolitan Museum of Art (fully available online as PDF), which contains material on yi