This video is from last week, and came in between the saddle going on, and Angi standing in the stirrup.
Prosperity still has some fear to work through before she will be ridden. It doesn't mean she won't be a GREAT horse to ride, just that Angi doesn't want to skip over this as it would only come back later, and probably at a really bad time.. Like in front of thousands in a noisy arena? Or worse after an adopter takes her home!
If the fear is there, it will come out at some point. So it's better to deal with it now, on the ground.
Prosperity needs to be able to handle being touched all over, while moving, before Angi gets on. She needs to be comfortable with the saddle moving, and the stirrups flapping. Most of all she needs to remember to look to her leader whenever she is unsure or afraid. For Freedom safety started as his ball. (he didn't need a leader) When ever he was unsure about something he wanted to go smoosh his nose into his ball.
Prosperity is good about looking for the answer much of the time, and as you can see while her reaction is very dramatic, the drama is short lived, then she turns to Angi for reassurance.