It can mean any species in the genus Hippocampus. Or if its a scientific listing of a specific species as ophiura said, not yet identified. And that could mean that it could later be IDed as an already known species or a new one. Or some may list it like that because they just dont know or wish to specify the species.
A site like the 2doctors list one like this and give it a common name of Black Seahorse. Yet they dont give a species name and I cant say why....but on that site they show a photo of, IMO, a Hippocampus kuda. SWF.com list their Red like this as well and IMO, thats Hippocampus erectus. But even I cant say for sure. And why they want say, IDK.
Fishbase list 45 valid names of Hippocampus spp.. With 12 new ones being listed in this decade alone.
You would have to post a quality photo of the species in question and perhaps then someone might could ID it.