Sea Pony Oddities
This is the place that you will find factory errors, minor variations from one sea pony to the next, and anything out of the ordinary.  I imagine this page will always be a work in progress, given how little is known about sea pony variations.  If you have any pictures you would like to submit, please email me!
MIP Sand Dollar
I purchased this pony from a seller located in Rhode Island.  Based on some discussion on the MLPTP, the best we can tell is that some extra ponies were sold either to Hasbro employees in Rhode Island, or at a Hasbro warehouse outlet of some kind.  She is packaged in a plastic bag marked "Made in Hong Kong", with her puffy sticker, ribbon, and a piece of plastic wrapped around her head.  She is identical to MIB Sand Dollar, without the shell, comb & brush, or box.
TE Fizzy on UK P&P Sun Shower card
TE Galaxy on UK P&P Ripple card
Obviously these were factory errors, and the wrong pony from Year 4 was placed on a sea pony back card.  Please note these were the UK exclusive Pretty and Pearly babies, so this was apparently a UK factory error.  I have only seen these two versions, as well as one with Gingerbread on Tiny Bubbles' card, so please let me know if you have seen others!
Possible Sealight Variations/Factory Errors
The Sealight to the left's hair is mostly orange, but there is a slight bit of pink in the lower half, about 1/3 of the way out from the roots.  The rest of the orange is solid, from roots to tips.  The roots are the same color as the tips, and there is no staining at all on the body of the pony.  The head is still factory sealed.  I received her from a collector who received her with already-orange hair. 

In addition to the hair color, she is a different body color than the typical Sealight.  The picture is also a comparison of three different body colors.  The darkest color is the traditional color, which is the color of every Sealight I have seen except for these other two.  I am inclined to believe the variance in body color is not fading, but I have no proof of this.

While I do not have proof that these are true variants, the fact that the color into the roots is so even in the orange-haired version, and the fact that the body colors on the lighter versions are so even, while their eyes and fin paint remain bright, certainly makes for interesting conversation.
Hair Thickness Variations
I have found two different types of hair thickness in adult sea ponies.  The standard appears to be a nice thick-haired version, with the pony's hair plugs very close together.  There is also a thin-haired version, with about half as many hair plugs covering the same amount of area on the pony's head.

The two Wavedancers illustrate this very well, with the Wavedancer to the left being thick-haired, and the one to the right being thin-haired.  While this appears to simply be a factory discrepancy, if you have a thin-haired version and would prefer a thicker head of hair to work with, you should be able to find one.
Back to Sea Pony Utopia!
Italian Maristella Variation
MOC Remco Sea Pony Fakie
This belongs to an ebay member outside the 'pony community', who has agreed to allow me to use the picture.
So far as I know, this is the only true variant sea pony.  Maristella was normally produced with yellow hair, but a few of her pink-haired variants do exist.  To the best of community knowledge, she only ever appeared in the Dutch/Belgian packaging, pictured below next to one loose. 
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