The default Prolog mode for GNU-Emacs can be activated by adding the following rules to your Emacs initialisation file:
(setq auto-mode-alist
(append
'(("\\.pl" . prolog-mode))
auto-mode-alist))
(setq prolog-program-name "pl")
(setq prolog-consult-string "[user].\n")
;If you want this. Indentation is either poor or I don't use
;it as intended.
;(setq prolog-indent-width 8)
Unfortunately the default Prolog mode of GNU-Emacs is not very good.
An alternative prolog.el file for GNU-Emacs 20 is available
from
http://www.freesoft.cz/~pdm/software/emacs/prolog-mode/
and for GNU-Emacs 19 from
http://w1.858.telia.com/~u85810764/Prolog-mode/index.html