Asset already includes several models (three fish models, one bird model and one butterfly model).
In fact, you can use any model, see the example "Fish Boids Danger", the shark is animated using a shader.
Creating Models in Blender
Place the cursor in the center (Shift + S -> Cursor to Center), create a cube.
Now this cube is the border, make sure that the size of your model does not go beyond the boundaries of this cube.
Center the object (Shift + S -> Selection to Cursor (Offset)) and rotate the object along the axes, as shown in the image.
Lift the model slightly.
Apply object transform (Ctrl + A).
Export the model to FBX with the specified parameters.