Apple's OS X is an amalgamation of several technologies, including the Unix-based Darwin OS. So it isn't surprising that a platypus named Hexley was chosen as the mascot. I recently happened across another platypus on my Mac, this time with a heart of perl.
Sveinbjorn Thordarson has created an open source tool called Platypus which allows scripting languages, such as Perl, to run as native desktop applications. It's very slick, and works like a treat. If you've ever wanted to use perl in desktop applications on a Mac, you should be running to give this tool a try.