A FON file contains font resources in Resource (.res) file format. The .FON format files were developed by Microsoft for Windows 3.x. These files comprise bitmap fonts utilized in early MS DOS GUI design.
FON files are EXEs containing only font resources, renamed with .fon extension. They are libraries instead of applications as font resources are not added to application resources.
FON files specify Windows 3.x font libraries referenced by programs accessing system fonts. They contain FNT format fonts and are now obsolete.
FONs are EXEs renamed to .fon containing stroked or bitmapped fonts as FNT files. Various programs using Windows fonts reference the default system FON file.
You may need to inspect FON files more closely to determine format as not all are TrueType. Most can open in Windows Font Viewer. TrueType FONs open in font editors like FontForge or Adobe Type Manager. FON contents viewable in text editors.
FONs have binary format and application/octet-stream MIME type. Microsoft has FNT format documentation.
Files with .FON extension are used in Windows to store bitmap fonts with predefined, unchangeable sizes. Some are preinstalled with the system and more can be added later. FON files risk infection so should be scanned thoroughly before system installation.
Microsoft Windows
FontForge