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The first half of Dave The Diver will be focused on you trying to get more information about an ancient civilization called the Sea People. Eventually, you'll learn that these people still exist and head to their village where the ruler will ask you to do quests for the villagers so you can get their trust.
These quests will appear in batches, and one of the quests in the first batch will ask you to retrieve four Seahorses that have been scattered all across the village. Here are the locations to find all of them easily.
Crowned Seahorse Location
As the name suggests, this Seahorse can be found in the crown room of the Sea People village. To get to it, you'll first have to use the portal at the bottom level right beside the mirror. This portal will take you to the upper part of the village.
Head to the right side and the throne room will be the first building you see. This is the same place where you talked to the Sea People ruler. As soon as you enter, the Seahorse will be swimming around with a blue marker on its head.
Dwarf Seahorse Location
Dwarf Seahorse is most likely the hardest one to find as it isn't in any obvious places, especially if you've finished all the Sea People quests in the first batch. To find it, you have to go all the way to the left side of the village on the bottom level. Here, you'll see a Seed Shop.
This is where you got one of the quests in the first batch, and you'll find the Seahorse floating around as soon as you enter. Simply swim up to the blue marker and press the required key as soon as it shows up.
Giraffe Seahorse Location
This is quite an easy Seahorse to find since it's in the open. You'll have to head to the same portal you used to get the Crown Seahorse earlier, except you don't have to use it this time. Reach the portal in the bottom part and you'll find it nearby.
It'll most likely be on the left side but it can float around to the top or the right since it's completely open. If you're missing this Seahorse, check the area around the portal thoroughly.
Hedgehog Seahorse Location
Finally, you can find the Hedgehog Seahorse in the open as well, and it'll be between the Seed shop and the house to its right where you met an NPC named Mima. This Seahorse can be quite hard to find because of its surrounding.
Even with a blue marker on top, there are a lot of colorful items in this area, which makes it quite easy to miss. Pay close attention and you'll get yourself the final Seahorse. Head back to Linchen and you'll unlock the Seahorse Racing mini-game.