What was the life story of Emperor Si Marui in Jin and Yuan Dynasties? Why do some people say that his original name was "Niu Rui"?

There is a story in the book of Liu Ji Yuan Di, the emperor of Jin Dynasty, that "the cow follows the horse". According to legend, there was a divination book Xuanshitu in the Cao Wei period, which recorded the divination of "cattle and horses courting prostitutes". Sima Yi asked Guan Ju, an astrologer, to tell his future generations' fortune, and Guan Ju's divination result was completely consistent with Xuanshi map. Sima Yi didn't know what this meant at that time. Later, he became a teacher, and his subordinates had a general named Jinniu. Sima Yi thought of the prediction that "the cow follows the horse", and his heart was very taboo. He was afraid that Taurus would be bad for his offspring in the future, so he made a hip flask with only one mouth with two kinds of wine in advance, poisoned the wine, and then sent someone to invite Taurus to dinner. Taurus died for no reason. In the early years of the Western Jin Dynasty, Sima Yi, the grandson of Sima Yi, attacked Duke Langya and took Xiahou as his concubine. Xiahou's romantic life soon hooked up with a petty official named Niu and gave birth. After the demise of the Western Jin Dynasty, Si Marui established the Eastern Jin Dynasty, which is called "Cattle and Horses". That is to say, he is not the real descendant of Sima, but the son of Niu who succeeded Sima to the throne, so later generations nicknamed him Niu Rui. For example, Li Zhi, a thinker of the Ming Dynasty, called the Eastern Jin Dynasty "the Niu Family of the Southern Jin Dynasty", not the Sima Family. In addition to the Book of Jin, there are many related descriptions in historical documents, such as Shu Wei written by Wei Shou in the Northern Wei Dynasty, Record of He Lin Yu written by Luo Dajing in the Song Dynasty, Hong Mai's Essays on Rong Zhai, and Zhao's Biography of Soldiers.

However, The Book of Jin Biography of A Yuan Xiahou Taifei has no record on this matter, only that Princess Langya is "a famous Guangji, and Pei Guoqiao was also born (noble), young and wise. Prince Wu of Langya (Sima Zhou) made a pilgrimage and gave birth to Yuan Di. So just talk and listen after dinner.