I have ridden some horses like that and I have found that the best way to deal with it is just to let them sit there and look at whatever they are frozen from. If they look at it long enough, they will realize that it isn't going to kill them. I usually urge a horse to go up to the thing that is so scary (providing that they aren't absolutely petrified of it) and have them sniff it. It will not be an immediate change but most of the time, they will start to unfreeze sooner and sooner until eventually, they don't freeze at all. I would always prefer a frozen horse over one who wheels and tries to bolt away from the scary object.