What is the possessive form of pony express?

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word:

• Pony Express'

Add an apostrophe s ('s) after the existing s at the end of the Word:

• Pony Express's

Examples:

The Pony Express' reputation grew quickly.

The Pony Express's reputation grew quickly.