As soon as the horses started down the track, Thunder Snow immediately pulled up and began bucking on the track. Whether it was the sloppy track conditions or something else, the horse wanted no part in competing in the Kentucky Derby. It led to plenty of concern about whether or not the horse was okay. With the broadcast staying on the race, it wasn’t until after that the status of Thunder Snow was learned. The on-call vet at the derby, Keith Latson, let reporters know that the horse is fine. The above was from this article. Kentucky Derby 2017: What happened to Thunder Snow?
I just watched ESPN which did a report on the Derby, they said the saddle/rider shifted on Thunder Snow which spooked the horse.
Thanks for that. I saw him bucking and the jockey trying to pull him up, but never heard anything later. At first I laughed, then I wondered if there was a problem but never heard afterward. The horse was from the UAE, so maybe wasn't familiar with mud. Hmm, if that happened then someone didn't do their job quite as thoroughly as needed. Maybe he forgot to poke the horse in the belly so he would expel air so they could get the girth nice and tight.