In higher (n-)category theory, there is not a single canonical way to describe higher categories, leading to lots of careful use of "strict" and "weak" qualifiers, as well as many different non-equivalent flavors of higher category: https://ncatlab.org/nlab/show/semi-strict+infinity-category
Speaking of categories, the number of finite categories, or other finite abstract algebraic objects, is not a beautiful combinatoric theorem, but instead a nasty study of computer-aided searches: https://www.mta.ca/uploadedFiles/Community/Bios/Geoff_Cruttw...
Classification theorems, in general, are frequently beautiful in their structure. But they often involve delving deep into rabbit holes, in order to prove the non-existence of the impossible.