Rat is a kind of animal that is familiar among humans, but due to the behaviors of stealing food and spreading disease, the cultural connotation and animal metaphors have become negative in almost traditional cultures. In the metaphor area, here are many comparative studies about animal metaphors of different animals in Chinese and English. Recently, new animal metaphors about "rat" in online language are also widely used, but there were scant studies about the new online animal metaphors. Moreover, there is no study that only focuses on animal metaphors about "rat," which is a research gap. This comparative study focuses on the difficulties for foreigners to acquire new online animal metaphors about "rat" and their cultural connotations in Chinese and English, along with the differences between traditional animal metaphors and new online ones. To find out the results, two new animal metaphors "Shu Ren" and "Jie Dui Zi" in Chinese and two new ones "Internet rat" and "RAT" in English are investigated. During the analysis, the origins and cultural connotations of these new online animal metaphors are found respectively and compared. The results show that new animal metaphors can be used as self-mockery instead of only describing others, and they appear due to the recent social phenomena. Moreover, the four factors of the difficulties for foreigners to acquire new online animal metaphors and their cultural connotations are society, politics, convention and language. Furthermore, this study can contribute to the area of metaphor for arising people's awareness of metaphor knowledge in animals, especially rats.