Which font file format is useful depends on the application being used. FreeBSD by itself uses no fonts. Application programs and/or drivers may make use of the font files. Here is a small cross reference of application/driver to the font type suffixes:
.fnt
.pfa, .pfb, .ttf
.pfa, .pfb
.pfa, .afm
.ttf
The .fnt suffix is used quite frequently. I suspect that whenever someone wanted to create a specialized font file for their application, more often than not they chose this suffix. Therefore, it is likely that files with this suffix are not all the same format; specifically, the .fnt files used by syscons under FreeBSD may not be the same format as a .fnt file one encounters in the MS-DOS®/Windows® environment. I have not made any attempt at using other .fnt files other than those provided with FreeBSD.