I searched on the forums and couldn't find anything related to this. I was messing around with a friend today and noticed that when I switched to stag form while he was in my mechano-hog it would dismount him and turn me into a stag on a mechano-hog. He was not able to mount up on me for a few minutes.
I went afk for a few minutes and when I came back I see this.
Others do see you like this as well until your friend is able to mount you again. (Although once your friend is able to mount you it looks like him on a stag.)
TLDR Version:
1. Have druid mount on Mechano-hog that has glyph of the stag.
2. Friend gets in the side car.
3. Druid clicks travel form.
4. Profit?