This goes back to ancient MSDOS. Certain names were 'pre-allocated' at every directory level - those names were basicaly I/O names - CON, LPT, COMx etc.
At one time there was a switch to allow those names to exist as long as they weren't in the root dorectory - no idea if this is still true or not.
Other folder names that you can't use
con (you know that already), nul & prn
If you want to create a folder called con, you can via the CLI, here's the command:
to remove the directory, here's the command:
I believe you can't use these names as they are used for batch entry.