Substracting the amount of total eqg, anthro and humanization images uploaded (numbers are taken from their respective tags) and current latest image ID (2093989), there would be around 1500000 images. While some of them would not fit in any category, be it characterless scenery, cutiemarks or else, there still would be more than a million of images without the "pony" tag.
Edit: plus, majority of new images are still being uploaded without this tag. I also forgot to exclude the pony tag itself, so there would be around 850000 images without proper clarification.
Edit: there are also plenty of images displaying other species, such as zebras or griffons, and literally no one would give them a "pony" tag.