Zhenwu, the Supreme Emperor of the Dark Heaven, is an important and widely worshipped deity whose origin can be traced to the late Warring States period and Han dynasty. He is originally represented by a tortoise entwined by a snake and was the ancient symbol of the North, also know as Xuanwu. For a discussion of Zhenwu see Stephen Little, Taoism, Chicago, 2000, pp. 291-292, where the author also illustrated a blue and white model of Zhenwu with a Wanli mark from the Museum of Fine Arts, Boston, in ibid, p. 297.
Also compare a fahua example with very similar modelling to the present lot, illustrated in He Li, Chinese Ceramics, 1996, p. 241