That's genuinely just the common way of modeling a tail with fur in lower resource environments. Not excusing it, the game is supposed to have a graphical update and I think they can do better than that.

But I think it's less of a mistake and more of a technique of its time that was harder to notice bank then. I hope that makes sense.

Modeling fur, hair, grass, etc and making it in a way that doesn't destroy the resources of your machine is pretty hard. That's why a lot of stuff is a flat texture that always faces the camera.

Here, it's just a few textures on planes intersecting with each other.