The G1 Merponies (Fancy Mermaid Ponies) are semi rare. I don't see them all that often, but they are not super expensive, so I would say they are not really popular. They run around $30-$70 depending on the auction at the time.
The G3/G3.5 are ponyville sized versions, and they aren't rare. They are later releases, and by then ponies were not popular, so they are rare simply because they weren't selling well in stores at the time.