Of thirty-seven varieties of seahorse on the International Union for Conservation of Nature (IUCN) red list of threatened species, seven are classed as vulnerable, one as endangered and the rest data deficient (not enough information) .
The entire genus Hippocampus was listed on CITES Appendix II in 2002. The Appendix II list covers species that are not necessarily threatened with extinction, but may become so unless trade in specimens of such species is subject to strict regulation in order to avoid utilisation incompatible with the survival of the species in the wild. 
Seahorse populations are thought to have been endangered in recent years by overfishing and habitat destruction. It is estimated that over twenty million seahorses are caught for use in traditional Chinese medicine, an unknown number for use in other traditional medicines, with an additional one million sold as pets (most of whom die) and another one million deliberately allowed to die, and to dry out, for the curio trade. 
It is suggested  that the use of seahorses in traditional Chinese medicine has grown due to the increasing prevalence of pills and powders over decoctions. Traditional pharmacists would ensure that seahorses met certain size and quality criteria before being used as medicines, but with pills and powders being produced in large factories, lower quality, smaller and juvenile seahorses are used.
Back in 1996, the World Nature Foundation estimated that world seahorse populations had been reduced by half and demand is thought to have increased dramatically since that date. 
Seahorses have low fecundity (out of a thousand young per seahorse couple per year, only two are expected to survive). Parental care is essential and since this is exclusively provided by the male, if males are taken, no offspring will survive. Seahorses are slow-growing (taking up to a year to mature), relatively immobile (therefore easily being caught), site faithful (so local populations can easily be eliminated) and sparsely distributed (making it hard for widows to find new partners). 
2. Convention on International Trade in Endangered Species of Wild Fauna and Flora, Appendices 1, 11 and 111.
3. Vincent, A.C.J. The International Trade in Seahorses. TRAFFIC International, Cambridge, 1996. UK ISBN 1-85850-098-24.