Things like rc files for shells and .mailrc for BSD Mail were simply scripts written in the program's own command language. The .newsrc used by ReadNews was an example of a common colon-separated key+value format, also seen in Xresources and others. The sendmail.cf used by BSD Sendmail was its own very special thing, indistinguishable from modem line noise.
BSD also widely used the "capabilities" file format, which still exists in many parts of FreeBSD, NetBSD, and OpenBSD to this day for various things such /etc/login.conf and /etc/gettytab and termcap. (In many cases nowadays, cap files are compiled into Berkeley DB or other constant database files by tools such as cap_mkdb and tic.)
Many configuration files in the Unix world were simply flat file tables, with colons, whitespace, or TAB as the field separator. phones(5) is one such TAB-separated table that is in FreeBSD to this day.