Please try to take this light-heartedly...
DD just started lessons. Other "Fun" things are FAR cheaper. Other organised activities I see the point: it's varied and social and fun (skating, Brownies), it's a valuable life skill (swimming) or there are obvious ways to measure improvement (Judo grading, music exams).
But horse riding... meh, it's not like she's got a goal to work for or even a friend's pony she could occasionally borrow. It's not an essential life skill or even one she's likely to ever use much.
So, you horsey lovers, can you persuade me to think differently?