It seems like emisissive textures are on some mobs such as the skeleton horse and the tarantula, it glows with shaders on.

Yes, we have emissives on the eyes of feral wolves, coyotes, hirschgeist, and tarantulas. I am aware of the shaders issue, but unfortunately it is not something I am aware of how to fix nor worry much about, as shaders and optifine often cause rendering issues and it should be expected. I will inform you should I ever solve this, but it is not something I will actively fix.