A pony is just like a horse, only shorter. Riding a pony can be less intimidating than riding a horse, because you sit a little closer to the ground. Giddy-up!
Many people think that ponies are young horses, but they're entirely different (though closely related) animals. A pony is smaller than a horse, with shorter legs, although often thicker and sturdier proportionally through the body. To pony up, on the other hand, doesn't have anything to do with equines: it means to pay money: "If you want a cup of lemonade, you'll have to pony up a dollar."