* Definition of "pony": There's no strict height limit that defines a pony. Some organizations use 14.2 hands as a cutoff, others use 14.0 hands, and some don't use a height limit at all.
* Breed recognition: Different organizations recognize different breeds, and some breeds are only recognized regionally.
* New breeds developing: Over time, new breeds can emerge from crossbreeding or selective breeding.
However, a rough estimate suggests there are over 100 recognized pony breeds worldwide. This is not including many regional or less recognized varieties.