Old versions of MacTeX are archived on the TUG server. To get one of these versions, use the "Go" menu in Finder, and select the item "Connect to Server:" Fill in the resulting dialog with the following address
A login dialog will appear; click "Guest". An ordinary finder window will appear showing folders with various dates. Open the appropriate folder, and drag an install package to the desktop. For example, the 2015 folder contains three items, basictex-20150613.pkg, ghostscript916-20150607.pkg, and mactex-20150613.pkg. Packages are dated by release date in the format year/month/day.
After obtaining the desired package, you must disconnect from the server. The ftp server will be listed in the left panel of the Finder window.
Select this server and click "disconnect".
Originally, MacTeX supported macOS 10.3 and 10.4, on PPC or Intel. Starting in 2011, MacTeX required 10.5 or higher, on PPC or Intel. The 2016 version of MacTeX is the last to support Leopard (10.5) and higher on PPC and Intel. MacTeX-2017 supported Yosemite, 10.10, and higher on Intel.
Early versions of MacTeX have a hidden folder structure, even though they appear to be single files in the Finder. These versions had to be compressed as zip files to be stored on the server. Later versions of MacTeX have a flat structure and thus appear as uncompressed single files on the server. Therefore when using ftp, recent install packages appear with a standard package icon, but older packages appear as .zip files that must be decompressed after downloading.