Pulao (蒲牢), known in some early sources also as tulao (徒劳), and Pu Lao, is a Chinese dragon, and one of the 9 sons of the dragon. It is said in Chinese mythology that he likes to "roar", and therefore he is traditionally depicted on top of bells in China, and used as the hook by which the bells are hung.
Pulao appeared in Chinese literature already during the Tang Dynasty. The Tang Dynasty scholar Li Shan (李善, 630－689), in his comments on Ban Gu's (32–92 AD) "Eastern Metropolis Rhapsody" (东都赋, Dong Du Fu), wrote:
In the sea there is a big fish called whale, and on the shore there is a creature whose name is pulao. The pulao has always been afraid of the whale. Whenever the whale strikes [or attacks] him, pulao cries [or roars] loudly. Thus those who want to make a load [bell] would put a pulao on top. Therefore the bell-striker would be made [in the shape of] a whale.
During the Ming Dynasty, the pulao (in the form tulao) appeared in the influential list of fantastic creatures appearing in architecture and applied art, which was compiled by Lu Rong (1436-1494) in his Miscellaneous records from the bean garden (椒园杂记, Shuyuan Zaji).
The tulao's form is like that of a dragon, but small. By its nature it likes to roar, and has supernatural strength. [They] used to hang bells [by the tulao]. 
- Quote from Li Shan's text, given in Yang Jingrong and Liu Zhixiong (2008): "海中有大鱼曰鲸，海边又有兽名蒲牢，蒲牢素畏鲸，鲸鱼击蒲牢，辄大鸣。凡终欲令声大者，故作蒲牢于上，所以撞之者为鲸鱼." In the article The Bell of the Bailin (Cypress Grove) Temple, the following English translation is offered: "There is the big fish called the whale in the sea and the animal called pulao at the seaside. The pulao is always afraid of the whale. Whenever the whale attacks the pulao, the latter roars. Whoever wishes to make a bell sonorous puts a pulao design on it. The striker is the whale."
- Quote from Lu Rong's Shuyuan Zaji, given in Yang Jingrong and Liu Zhixiong (2008): "徒劳其形似龙而小，性好吼叫，有神力，故悬于钟上。" The full text of Shuyuan Zaji can be found at a number of sites online, e.g. here: 菽園雜記
- Yang Jingrong and Liu Zhixiong (2008)
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