That’s a very pretty pony and the fancy fronds that adorn its head and back really give it a fancy appearance.
Those hair-like tentacles on the head and neck are known as dermal cirri, and are an attractive adornment possessed by certain seahorses. Dermal cirri are fleshy tabs or branching outgrowths of the skin that serve to break up the seahorse’s outline and allow it blend into its weedy habitat all the better, a sort of natural camouflage. Unlike spines, cirri are not permanent structures in most cases. Up to a certain age at least, seahorses appear to be capable of growing or shedding these fleshy filaments as the occasion demands in order to better suit their surroundings. For example, specimens that are rafting in clumps of Sargassum are apt to have well-developed cirri, giving them an appropriately shaggy appearance, while a seahorse inhabiting the mudflats of an estuary will be smooth skinned. Cirri grow most commonly on the head and neck region and are more common in juveniles than adults.
The presence of cirri is a highly variable trait and some species never have them. They are very rare or nonexistent in many seahorses, while in other species they are relatively common. For example, Hippocampus comes and H. reidi are smooth-bodied seahorses that never seem to develop cirri (Giwojna, Oct. 2003). Consider them cue balls — the Kojaks of seahorses. On the other hand, Hippocampus guttulatus are famous for their cirri and many Pot-Bellies (H. abdominalis/H. bleekeri) also sport fancy headdresses (Giwojna, Oct. 2003).
But even in seahorses where cirri are not uncommon, such as Hippocampus zosterae and H. erectus, the occurrence of cirri varies greatly from individual to individual (Giwojna, Oct. 2003). Most dwarf seahorses have no cirri, but some of them are regular little fuzz balls. That’s the case with Hippocampus erectus as well. Most Lined Seahorses (H. erectus) lack these appendages altogether, some have just a few, and the individuals with really extravagant cirri are relatively rare (Giwojna, Oct. 2003).
It’s a shame seahorses with well-developed cirri aren’t more commonplace because they can be quite breathtaking. A heavy growth of cirri can transform an ordinary specimen into a real show horse, making them appear as if they were adorned with a fancy mane or wearing an Indian war bonnet. A seahorse with extravagant, well-developed cirri can indeed be very exotic looking, but sometimes it has the opposite effect, lending them a comical appearance instead (Giwojna, Oct. 2003). I’ve seen shaggy specimens that looked like they were having a bad hair day, sporting a Mohawk or spike hairdo (Giwojna, Oct. 2003). Voila — a punk-rock pony, going through its rebellious teenage phase! Either way, they dress up the seahorses and give them a little extra pizzazz, and that’s what makes seahorse keeping so much fun!
Unfortunately, in this case, the heavy growth of cirri on the head of your seahorse obscures the coronet and make it difficult to identify with any certainty. I agree that it does not appear to be Hippocampus barbouri — the snout is not striped and it lacks the sharp spines that are so characteristic of this species. Neither is it H. reidi, H. kuda, H. comes, H. ingens or H. kelloggi.
The seahorse has the robust, deep-chested body of Hippocampus erectus, and erectus is one of the species that sometimes developed extravagant cirri, but it lacks the lined coloration pattern that is so characteristic of H. erectus. It’s not uncommon for the lined pattern to be obscured or difficult to detect on some specimens of H. erectus, though, so we can’t rule out that identification.
I might be inclined to think it was a specimen of H. guttulatus, which are so well known for their beautiful cirri, but we never see that species on this side of the pond.
I’m sorry to say that I can’t identify the seahorse from your picture with any degree of certainty. I believe the dealer you got the seahorse from called it a "spiny seahorse" because of its well-developed cirri, and not because of its spiny exoskeleton, which is why common names are very often misleading and useless for purposes of identification.
In any case, it’s a very beautiful seahorse and I would suggest trying to obtain another specimen of the same kind from the same source where you got this pony.
Best of luck with your new seahorse, Grant!