It's Argulus, a crustacean parasite, also called fish louse. They are quite common with pond fish (fresh and saltwater).
They can be manually removed (use tweezers) if just dealing with a low infection on a few fish in a QT setting.
Chemotherapy to treat larger systems with many fish would require Trichlorfon (Dylox).
I've seen aqua cultured seahorses come with these - it seems the breeder has an infested outgrowing pond.
You will want to do a medicated dip of some kind to clean them off your horses. Google it for suggestions on how to proceed. Removing them manually is only a temporary solution as there are always more that you don't see in your system to reinfect your horses.
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