If you don’t know how to share a color between 2 meshes perhaps hair cards are a bit too far ahead in your learning path.
That said. A hair card is literally just a piece of geometry with up to around 12 verticis thats bent about in order to provide visibility.
this geometry is then layered over the model however you see fit.
In blender, you can use a hair emitter, a mesh with an instance modifier and align to path checked, and brush a quick sculpt away - with hair length control so you can add more variation.
the base tris count of the hair card, as well as its quality (set on the hair emitter) determine the overall end cost of the mesh in tris.
The hair rendering needs a flow map to look good in Unreal. Not sure if or how to output one in Blender. Essentially you will need a custom system to get world normals onto the vertex colors, since it needs to travel with the mesh - again, if you don’t know how to get a mesh to share the color of another mesh, this is well out of your understanding - he11, its out of mine…
You can check the UE4 hair rendering docs and the hair cards content example for a quick material / base start.
Depending on your animal, a basic shell and fins implementation may look better and possibly outperform cards.
Also, the new hair and fur stuff wirh grooms - though it definitely does NOT outperform hair cards - can be made to look good.