The Mac's batch rename feature is hiding in plain sight inside Finder. Select multiple files, right-click (or Control-click), and you'll find "Rename X Items" in the menu.

Mac OS X computers use a default application to open every file type based on its file extension. If you don't want to use the default application when opening a file, you can select "Open With ...

If you ever find yourself needing to rename several files at once, Mac OS comes with several inbuilt options for getting the job done.