Default fixed fonts are located in /usr/local/lib/X11/fonts. I think /usr/local/lib/X11/fonts/misc are of interest. Try
ls /usr/local/lib/X11/fonts/misc
You will see a lot of font files, along with "fonts.dir" and "fonts.alias" files.
In Your .Xdefaults You can use either font description from "fonts.dir", for example
-misc-fixed-bold-r-normal--18-120-100-100-c-90-iso10646-1
or alias from "fonts.alias", for example
10x20
Sorry, may be it is not the best explanation, but I feel that You understand.