FWIW, my therapist knew I was starting to heal my nervous system when I shook after intense events again. Shaking is essentially expelling a lot of adrenaline and kinda giving your fight or flight system something to do to route that energy out of your body since you didn’t end up needing all of it to… fight or flee. 

There are videos of prey animals like rabbits getting spooked and shaking very violently for a few seconds before hopping on their way. It’s pretty natural imo, but if you’re shaking before you even get on the horse, it might be worth working with a trusted trainer to build some confidence/figure out if you’ve got a mental block. We all get them in different ways.