I am a promise that makes the world run, the more you use me, the more you have fun. What am I?

Question

I am a horse that will not run,
And in the dark I won't be fun,
I cause a stir, perhaps a scream,
To have me you will not be keen,
You'd like to keep me on a tether,
But you wont escape my appearance forever.
What am i ?

Answer 1

It is money.