It will cost TOO much.
It becomes a lifestyle that you will grow to hate over 20 years while it's interfering with everything else.
Car will start to stink of horse feed, family members will start to smell of horse poo and pee. Wellies will be in the porch stinking of the same.
Car will need to be changed so it can tow a horse box that you will need to buy and store somewhere. Vet fees will take away your beer money.
The list goes on.
Eventually if you're lucky after a few changes of the horse for a bigger one it will need to be sold along with everything else when the rider discovers boys and makeup and going out in dresses, that's if you're lucky. If not, then she will carry on spending your money because no self respecting lad will be going out with a girl that stinks of horse pee and poo most of the week. If she isn't smelling of horse pee and poo then that means your paying for mucking out instead of her doing it.
Cold winters at the stables, lifts to horse events, etc.etc.
Worrying about your daughter riding on the congested roads with lunatics behind the wheel.
Worrying about life changing injuries because horses are unpredictable while on them or around them. Politics at the stables causing bitching and moaning.
Just don't entertain it, steer well clear and keep horses out of the family dynamic or you will live to regret it.
Put your spare money into a savings account for her and buy her driving lessons and a car when she's old enough and a contribution to a deposit on her first house, it will cost less over the long term.