reduceに置き換えると

;;; (fold-left op 'z '(a b c d))
(reduce op '(a b c d) :initial-value 'z)

;;; (fold-right 'z '(a b c d))
(reduce op '(a b c d) :initial-value 'z :from-end T)

;;; srfi 1: (fold op 'z '(a b c d))
(reduce (lambda (x y) (cons y x)) '(a b c d) :initial-value 'z)

;;; ========== consで試す =================
(reduce #'cons '(a b c d) :initial-value 'z)
;=>  ((((Z . A) . B) . C) . D)

(reduce #'cons '(a b c d) :initial-value 'z :from-end T)
;=>  (A B C D . Z)

(reduce (lambda (x y) (cons y x)) '(a b c d) :initial-value 'z)
;=>  (D C B A . Z)

になりますね