I will buy that maybe, just maybe, the purchase of six ponies was (1) not considered too unusual, (2) not too costly for our small group of conspirators. As evidence I note that, upon learning that there are six ponies stabled across the field, Frodo doesn't exclaim, "My word! However did you come across so many, and where did you find the funds?"
That said, I will not buy that ponies were very ubiquitous. Some of the posts above had me envisioning vast grassy plains like those found in the old American West, where the buffalo roamed in herds in the millions - but in this case it would be ponies.
Where would one get the fodder for such a four-legged army? None of the inhabitants of Bag Hill were seen riding ponies. Frodo, when he considers whether to ride or to walk to Crickhollow, chooses to walk. Had he decided otherwise, he would have ridden *behind* a pony, not atop one. The few, if any ponies seem to be work animals and not part of the general transportation system.
And for a community to produce ponies in excess would put stress on the system, and obviously, of course, attract wolves (as we see later in FotR).
Not sure any of that is worth any silver pennies...