I have attached some photographs of this, I believe, a tankard. It has no marks on its base but rather a seal on its body. The details on the monkey's face is excellent. I would appreciate your opinion.
Your tankard is Japanese "Sumida" pottery. Sumida Pottery was made for export to the West and is usually heavy and covered with figures in relief. Most pieces are everyday objects such as tea pots, vases, and mugs. This distinct type of wares got its name from the Sumida river running near the Asakusa pottery district near Tokyo.
The style of applied figures on a surface with flowing glaze was invented about 1890 by the Seto potter Ryosai I, who worked in Tokyo from about 1875 to 1900.
They are often embellished with glazed plaques with hand written signatures or general good luck symbols.Your mark is a signature, saying "Inoue Ryosai", Possibly Inoue Ryosai (1888-1971) who moved the manufacturing site to Yokohama in 1924.
A great number of the pieces are probably the work of a single family - Inoue Ryosai I (1828-), Inoue Ryosai II (born c. 1860), and Inuoue Ryosai III (1888-1971.)
The style is continued until today and I unfortunately don't know enough about this kind of pottery to suggest any specific date for your piece, or have any opinion about its authenticity. Since a mug like this, genuine and of good quality, are worth about $200 it might be worth the effort to investigate this further, though.
I hope this helps some anyway :-)
Thank you for your interest.