I found out that if you use the macro-taming method on a warp-stalker, using a freezing trap, it will have an ability called 'Phasing Invisibility'... This will proc every 30 seconds, turning the pet invisible for 8 seconds.
It will just sit there and do this by your side forever!
Here's the catch, though. (...and this is a good catch, IMHO)
If you attack anything with him, he will lose the buff. Forever.
This is good because it means it can't be taken advantage of by others and is purely cosmetic!
Now I know a lot of you will say this is useless, as it doesn't change his appearance per se... But I found this out and thought it was neat. I thought up a cool back-story for him, and he is only for show and role-playing.
