Names of specific entries
A specific entry is not the same as a proper noun. Some proper nouns can be pluralized, while a specific entry cannot. Specific entry refers to something there is only one of.
Specific entries must be attestable with at least one generic meaning. Peter Pan for example is also used to mean "a man who acts as a child and does not accept the responsibilities of adulthood.".
Note, there is no consensus about whether to apply these rules to place names or not.