Hello I have checked a couple of furry animals models in skyrim and I see that they are using a strange system for the fur , the diffuse texture has an alpha texture that looks like a kind of specular map , but it is used both for the main body nif mesh , without the alpha property and als for the semitransparent alpha meshes to make the furry extrusions of hairs ... now the system ingame looks good but when I tried to make similar things I ended up with a very bad result because the texture was taking a shaded plane alpha instead than cutted pieces of fur hairs as is for bethesda ... now there must be some conditions missing ? or what? how the fur of beth work?

The furry extrusions are UV mapped to coincide with the alpha channel of the texture. If you want to copy that you need to carefully stay within the UV map for your new texture.