The Seahorse Boss fight is the big conclusion of the Bubble Wand area, which is the southwest section of Animal Well. Defeating this boss and claiming its flame is actually one of the more simple and mellow battles in game. While defeating this boss doesn't require any tools, reaching this area does require you have the Bubble Wand, Disc, and Yoyo, two of which can be found in other sections of Animal Well.

Seahorse Boss fight

You will drop down into this area from above, with the flame in the room to the right. In order to open the door on the right, you need to redirect water into the three pillars. The Seahorse will pop out of the water and shoot water towards wherever you're standing, so you will need to position yourself so the Seahorse sprays one of the two platforms you can move using the cranks.

There is a line of ghosts over the water below you, so if you fall into the water you will take damage. The Seahorse's water spray will push you slightly, although if you are actively using a crank you don't move. It seems as though you can use the platforms closest to the pillar to angle it in, with both platforms working for the middle pillar. Once all three have had enough water, the door to the right will open and you can collect the flame.

The Seahorse Boss is located in the room with the red death skull in the above image. While you don't need a tool for the boss puzzle itself, you do need the Yoyo, Disc, and Bubble Wand to reach this area.