Assalamualaikum ustaz. One of my friends suggested to grind seahorse and add some of it into my drink to treat my asthma. Hence, the question that arises is what is the ruling of consuming seahorses? Thank you.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
We begin with the statement of Allah SWT:
أُحِلَّ لَكُمْ صَيْدُ الْبَحْرِ وَطَعَامُهُ
“Lawful to you is game from the sea and its food,”
Syeikh al-Sa’di stated that while game animal includes all game animals on land and in water, Allah SWT exempted for you – when you’re in ihram – game of the sea and dead aquatic animals found. Thus, this shows that the carcass of aquatic animals is halal. Furthermore, the wisdom of them being halal is for you to benefit from as well as your companions who are with you on a journey. 
Definition of seahorse
A seahorse is a sea animal whose head resembles the head of a horse, the Hippocampus hudsonius.  While in the World Encyclopedia, it is stated that a seahorse is a small fish that is so named because its head resembles a small horse. About 25 species of seahorses live in shallow tropical seas. Some species are also found in temperate climate waters. Most seahorses are less than 15 centimetres long. One species of seahorse from the Pacific region can reach up to 30 centimetres in length. Seahorses belong to the same family as tap fish.
The body of a seahorse is constructed of a bony plate. This fish has a long snout and feeds by sucking small animals into its cylindrical mouth. The tail is long, flexible, and can grip onto things. Seahorses use their tails to mount on rooted plants or floating sea plants. Seahorse foals often form small groups by clinging to each other with their tails. The large swimming pool on the seahorse contains air and allows the seahorse to be at a certain depth. Seahorses swim slowly using the hind fins and pectoral fins. The pectoral fins are located on the side of the head that looks like a pair of ears. Seahorses have an incredible way of breeding. Male seahorses have special pouches where females lay their eggs. The stallion then frees the delicate child. 
However, factors such as global warming as well as unfavourable environmental conditions have resulted in seahorses facing the threat of extinction, even more so if there is no effort to protect this sea life. Thus, the Johor State Biotechnology and Biodiversity Corporation (J-Biotech) has become the first agency to conduct research on the conservation of seahorses and their environmental ecosystems since 2008. J-Biotech’s Chief Operating Officer, Azman Mohamed Shah, said that the biodiversity research plan was drafted in collaboration with Universiti Malaysia Terengganu (UMT) and opened a new chapter in the marine conservation program in Johor which includes research and commercial elements.
He added that the development of the first phase of seahorse conservation research that is being done is comprehensive scientific research through breeding and rearing programs, rearing them in an artificial environment and returning them to their original habitat. While the second phase involves downstream research which is an effort to extract bioactive compounds in seahorses for medical and clinical purposes that are being worked on and developed. This is because seahorse compounds are seen to have the potential to help treat cancer, tumours, impotence, blood clots, kidney toxicity, asthma, skin treatment, anti-ageing anti-fatigue compounds, the immune system and the nervous system. 
The ruling on seahorse
Basically, aquatic animals are divided into two categories which are the fish species where their physical characteristics are well known. It is halal to be eaten, regardless of how it died and it isn’t necessary to slaughter it. This is based on the statement of Allah SWT:
أُحِلَّ لَكُمْ صَيْدُ الْبَحْرِ وَطَعَامُهُ مَتَاعًا لَّكُمْ وَلِلسَّيَّارَةِ
“Lawful to you is game from the sea and its food as provision for you and the travellers,” 
Likewise, there is a narration from Ibn Umar R.Anhuma, he said:
أُحِلَّت لَنا مَيتَتانِ ودَمانِ؛ الجَرادُ والحيتانُ، والكَبدُ والطِّحالُ
“Two types of animals which have died a natural death and two types of blood have been made allowable to us, the two which die a natural death being the fish and the locust, and the two types of blood being the liver and the spleen.” 
Second, aquatic animals that don’t have the same physical characteristics as fish, such as capybara and seal. Scholars have differing opinions on this category. In this category, Imam al-Nawawi cited three famous opinions regarding this matter in his book and stated that the sahih opinion among scholars of madhhab al-Syafi’I and all of them said it is halal to be eaten for the word al-Samak “السَّمَك” – as stated in the hadith – refers to all aquatic animals. It is also included in the statement of Allah SWT:
أُحِلَّ لَكُمْ صَيْدُ الْبَحْرِ وَطَعَامُهُ
“Lawful to you is game from the sea and its food as provision for you,” 
Likewise, there is a narration from Abu Hurairah RA, where Rasullullah PBUH said:
هُوَ الطَّهُورُ مَاؤُهُ، الحِلُّ مَيْتَتُهُ
“[Regarding the sea] “Its water is purifying and its dead (animals) are lawful (to eat).” 
According to the above discussion and arguments, in our opinion, a seahorse is permissible to be eaten for it is included as an aquatic animal which is from the fish species that doesn’t have the usual appearance of a fish. Thus, both seahorse and its carcass are halal according to the general evidence of the Quran and Sunnah.
Especially the fact that shows seahorse is eaten and considered a delicacy in certain regions of the world. 
May Allah SWT grant us all a clear understanding in religion. Amin.
 Surah al-Ma’idah: 96
 See Tafsir al-Sa‘di, 1/244.
 See Ensiklopedia Dunia, (Terj. DBP), 11/560-561.
 See https://www.bharian.com.my/bhplus-old/2016/05/150349/ubat-kuda-laut-penawar-mujarab. Accessed on 4th February 2022.
 Surah al-Ma’idah: 96.
 Narrated by al-Baihaqi (1211) and he stated in al-Sunan al-Kabir, the sanad of this hadith is sahih.
 See al-Mu‘tamad fi al-Fiqh al-Syafi‘i, 2/547-548.
 See al-Majmu‘ Syarh al-Muhazzab, 9/33-34; al-Mu‘tamad fi al-Fiqh al-Syafi‘i, 2/548.
 Surah al-Ma’idah: 96
 Narrated by Abu Daud (83); al-Tirmizi (69) and al-Nasa’i (59)
 See https://catchandfillet.com/can-you-eat-a-seahorse/ and https://fishmasters.com/can-you-eat-seahorse/. Accessed on 4th February 2022.