snothouse From Wiki:
"The original cast for the film included Joan Rivers in Bobcat Goldthwait's role. Elliot Gould was the original voice of the horse. After a poor test screening of the film, the horse's half of the script was rewritten by future Monk creator and executive producer Andy Breckman in an effort to make the film funnier. John Candy was hired to re-record the horse's voice; he ignored the new script and improvised the dialogue instead."