I have seen gnimo several times standing around without anyone near him who owns the mount. So I would say he just did not despawn after someone unmounted. First time I saw it was on icecrown just at the border of some phasing area. But today there have been two standing around just in Dalaran in front of the bank and in Ironforge at the door to the portal room. So I was curious if there may be a known bug how to enforce this. Or is it really just a random bug that may have to do with server lags or something? Anyone got information about that?