I doubt it holds any truth. Do you fart when your excited? No. You fart because there is a build up of air/gas in your belly. They may fart more when they are more stressed/anxious, I know lots of horses will poop more when they're nervous. And even then, it's an internal thing.
My horse farts when he is nervous about something, like if he's in an arena alone and there is an approaching storm he'll start farting. He's kinda like me. I have anxiety problems and when I get nervous or anxious about something I get really gasy. Posted via Mobile Device
I seem to agree with the nervous or anxious part. Because I rode in my district horse show yesterday and my horse is an extremely hot horse. And she never ever passes gas the way she did when I was about to take off for barrels. She wouldnt stop. I was kinda embarrassed