Question: Was "Dakota" really the secret word the money carrying horses trained to stop for?
Answer: It's unlikely that code words were used like that in real life; horses don't understand "words", just sounds. Expecting a team of panicking runaway horses to respond to a separate code word that means stop, when their regular voice command for stop ("ho" or "whoa") is ineffective just doesn't make sense.
They weren't panicked runaway horses, as stated in the film, they were money carriers trained not to stop no matter what. Regular voice commands weren't effective because they weren't trained to obey them. The code word was the voice command. And horses can, in fact, be trained to respond to specific words, so it is entirely likely.
Answer: In the movie, yes, it was. Historically, nobody knows as different teams of horses were most likely trained to respond to different code words. The code word "Dakota" was just an example the movie used to illustrate that code words were used for stagecoach deliveries, especially when the coaches were transporting high-value cargo.
Scott215