A recurring joke on Crash Course World History, in which the presenter would make some broad generalisation (Usually about empires, Asia, or warfare) and then a brief montage of Mongol-related imagery would be cut into to point out that the Mongols are the exception to whatever generalisation it is.

Usually, of course, the generalisations are themselves bad, or there are numerous "exceptions." It's a running joke, not a statement of fact as such.

I first heard it on YouTube channel crashcourse world history. It means that the Mongol empire was unlike any other empire before them or after. For example, they did not rely on naval superiority like the Romans and British empire