Seaponies were readily available on the fakie market in the wake of the MLP movie (2018). I have found some different varieties.
Exhibit 1 is pearly seaponies that match horn/wing configuration of Hasbro, but have the Hasbro imprint removed from the tail (the big CE is still there, but all the smaller print is gone).
The materials and quality are virtually identical to the Hasbro ones, that makes me wonder if they’re even coming from the same factory.
Exhibit 2 is pearly seaponies that have different horn/wing configurations from authentic Hasbros, but interestingly these DO have the Hasbro markings (the ones I have, anyway). Again, the look and feel of the plastic is identical to Hasbro. (Alternate Pinkie not shown.)
Most of the time they are sold loose without packaging, but my unicorn Rainbow Dash came in a box with fakie mini figures.
Exhibit 3 is regular-finish seaponies. These are made of lower-quality plastic and have no Hasbro markings; they are more easily identified as fakies. The Mane 6 along with four other seaponies (Queen Novo, Princess Skystar, purple/pink, and white/yellow) were made in this group. (Rarity not shown.)