Lol, make him look like a happy spaniel :D My opinion on the tail is that it's should look like it's an entity of it's own, like a snake glued to the xeno's butt. If you think of the tentacles of Doctor Octopus in Spiderman, like an evil "snake brother" behind the xeno's back just waiting to strike. Sadly this is not the case in most places, if you look at alien: isolation the tail is often like a cross between a rhythmic gymnast's ribbon and soggy spaghetti. But if there was a real-life xeno I know it would use the tail for balance when moving.