Generally speaking, the celebs’ secrets for thin legs are:

LOTS of yoga/pilates. We’re talking hours at a time with a personal trainer over a long period of time. Also they’d probably tend to stay away from any activities that will bulk them up such as strong weight resistance or, as you said, horseback riding, as that will cause the muscle mass to increase.

Cosmetic surgery.

Eating disorders. Starvation burns as much lean body mass (muscle) as it does fat, so when a celeb goes anorexic it’s not just the fat in their bodies that they’re losing, and this is one reason why a lot of people with EDs have bad heath and heart problems – the heart is a muscle and losing “muscle mass” weakens it.

I totally get where you are coming from. I too am a horse lover and i am a horse trainer! I train TB and also do harness racing.. its alot of work as you will know and its hard work on the legs.. my muscles were huge and bulky but i did something about that.. i started to run a bit.. early in the mornings are better!! just light running dont strain your muscles at first impact.. then few days in invest in a swiss ball.. it does u a world of good.. do leg excessive leg workouts.. i use Gunnar Peterson core secrets which i paid out for.. it is quite expensive.. it has all the routines u need to do.. Goodluck i hope i was a little bit of an assistants to u. Ange

It’s called anorexia.
Don’t do it. My sister is a horseback rider and her legs are amazing. Muscles are ****. Bones are not.