Honestly, not much is known about the Hippocampus' abilities. Obviously, it has the abilities of a fish, meaning it can swim through water and breathe under water. It also has the ability to lay large, transparent eggs where its foals are born, but it is unknown whether these can be used for potions or other. Its foals are called Tadfoals, and it is known that it can be domesticated, as it was in 1949, meaning that you technically can ride it.
The Hippocampus might be one of the more difficult creatures to hold as a pet. Seeing as they are the size of a horse, they would require a large amount of water to be in, meaning that you would require a rather large lot to hold them. I would imagine that they would be rather pricey to hold too, seeing as they most likely eat a lot of water plants. If you are interested in creatures like this, you need to be dedicated to socializing and becoming friends with them, especially if you wish to domesticate the creature and use it as a horse. Moving the creature could also be hard, meaning that it could be hard to get your hands on, but also if you wish to breed them, it could be hard to sell young Tadfoals. This pet is definitely one for the more experienced pet holder, who knows what they are going into.