Origin of word / term - 怪獸?
Although the current usage of 怪兽 is dominated by the Japanese 'kaiju' concept, the word is probably not a Japanese creation. 

Sima Xiangru (司马相如) used it in one of his works, 封禅文 (2nd century BC):


  然后囿驺虞之珍群，徼麋鹿之怪兽，䆃一茎六穗于庖，牺双觡共抵之兽，获周馀珍、放龟于岐，招翠黄、乘龙于沼。


The modern word 怪兽 was probably reintroduced into Chinese by Japanese filmmaking, as this word only shows up in Chinese after the late 1930s, which coincides with the emergence of the Japanese kaiju movies. 

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