These ponies are based on the "seapony" incarnations of the Mane 6 characters, as seen in the My Little Pony: Friendship is Magic movie. During the movie, the pony characters are transformed into fantasy pony-seahorse creatures. Seaponies were first seen in the G1 cartoons, but had much simpler designs than the G4 seaponies do. Though these ponies were all found separately at various thrift stores, they originate from a 6-pack featuring all the main characters from the movie. This 6-pack was an Amazon (the online retailer) exclusive. Very similar incarnations of these figurines were also released in single packs, with a key difference between the multi-pack and single-pack releases being that the single-pack releases have a smattering of glitter on their backs. The toys from the single-pack line are referred to by the name "Glitter and Style Seapon[ies]".
As you may notice, my collection is missing Rainbow Dash and Rarity.
The flighted ponies have had their wings mutated into a big, translucent pair of fins on their dorsal surfaces. All of the ponies have their cutie marks present on a fin on near the base of their tails. Their tail fins match mane/tail colors of the ponies' normal horse-shaped designs. Their tail fins turn on a joint, which can swivel side to side and wiggle up and down. It seems best to use the tail as a base to stand the ponies up, as the joint on a few of them feels very tight, like it could be easily broken or bent out of shape if posed too much. All the ponies have pretty nice feeling hair, by modern My Little Pony toy standards. Not the greatest quality hair, but far from the worst. All of these ponies have huge, vibrant, inset eyes. Their irises are glittery, and the plastic insert makes them look glossy and three dimensional. A bit of a different look for a My Little Pony, but overall, it stands out and makes them look appealing, in my opinion.
The seapony-ified Mane 6 were featured quite prominently in the My Little Pony movie, likely as an excuse to put new Pony toys on the shelves. Seaponies are, essentially, the mermaid version of the My Little Pony characters. Mermaids are to be a popular subject for toys marketed towards young girls. Many popular doll lines have mermaid-themed releases of characters that are not otherwise mermaids (Barbie on multiple occassions, Monster High's Great Scarrier Reef, Bratz' Sea Stunners, etc.). Lines like Mermaze Mermaidz (and many others) exclusively feature mermaid dolls.
Lots of new seapony toy releases were unleashed onto toy aisle shelves coinciding with the release of the movie. The seaponies, however, seem quite rare in thrift stores, when compared to regularpony G4 toys. I also see little fan discussion of these figurines. They seem to have a quiet presence in the real-life and online spaces I frequent. All these factors point to the seaponies not having a huge impact on Pony fans, but I can't prove this. Seaponies are not new to the MLP franchise-- they first made their appearance in 1983. G1's seaponies were cute in a weird, dumpy way. They were lumpy and strange, lacking a lot of the intensity and detail that the G4 seaponies have. They harken less to the image of a marketable fantasy mermaid than G4's seaponies do. A few features come to mind when thinking about toy mermaids-- namely their big, colorful tails and fins, which G4 seaponies possess and G1 seaponies do not. In my eyes, the G4 seaponies say "mermaid" much more than the original seaponies do. It is mysterious that there is so little fanfare around these ponies. It could also just be due to the decrease in overall quality of the G4 toys towards the end of the series' life, though this set of ponies in particular is not of terrible quality. Having bought them from the thrift store, I enjoy these figurines, and wouldn't mind finishing my collection, should I happen to come across them again.