As a biologist, and an avid collector of sea creatures in jars, I feel I can tell you the best way to preserve the seahorse (though I'm unsure about how to do so with magickal intent). Firstly, it depends on whether you want to dry the seahorse out- or if you want to keep it in a jar so that it looks as it did when alive.
To dry the seahorse out: Soak it in rubbing alcohol for a few hours. Once it has soaked, take it out and either place it in direct sunlight. You can also dry it gently with paper towels. If it has issues drying, or there's not a lot of sun exposure where you live, you can also add salt onto the seahorse to quicken the process. The tail might bend as it dries. If so, you might hold it down with a weight or place the seahorse between two plates.
To preserve the seahorse in a jar: It's always best to have a preservative like ethanol, but that can be hard for people to get sometimes. If you go to the store and buy 90% rubbing alcohol (the highest you can buy in the store) this can work just as well. Fill the jar with the rubbing alcohol, place the seahorse in, then seal the jar air tight. Keep in mind that the more you fiddle with it and open it, in the future, the more you might see some decay. This typically will work for a long period of time. I currently have a Portuguese man o war, a baby shark and a puffer fish all preserved in this manner on my altar.