gistfile1.rkt

;; goes in .racketrc
(define current-cow-flags (make-parameter ""))
(define old-print (current-print))
(define (print-with-cowsay val [port (current-output-port)])
  (define output (with-output-to-string (thunk (old-print val))))
  (when (not (string=? "" output))
    (match-define
     (list in _ _ _ _)
     (process (format "cowsay ~a \"~a\"" (current-cow-flags) output)))
    (display (port->string in) port)))
(current-print print-with-cowsay)
(define old-error-display-handler (error-display-handler))
(error-display-handler
 (λ (string val)
   (parameterize ([current-cow-flags "-f flaming-sheep"])
     (print-with-cowsay string (current-error-port)))
   (old-error-display-handler "" val)))

gistfile2.txt

$ racket
Welcome to Racket v5.90.0.4.
-> (list 1 2 3)
 ___________
< '(1 2 3)  >
 -----------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||
--> (/ 1 0)
 ______________________
< /: division by zero  >
 ----------------------
  \            .    .     .   
   \      .  . .     `  ,     
    \    .; .  : .' :  :  : . 
     \   i..`: i` i.i.,i  i . 
      \   `,--.|i |i|ii|ii|i: 
           UooU\.'@@@@@@`.||' 
           \__/(@@@@@@@@@@)'  
                (@@@@@@@@)    
                `YY~~~~YY'    
                 ||    ||     
;  [,bt for context]