There is some debate as to whether this mob shares a spawn point with the Ironfur Great Bulls and Ironfur Grazers, in which case you might be able to force the rare to spawn by mass killing those.
However, I don't think this is the case. The rare does not spawn in all the places the other Ironfurs do (such as the backside of the Celestial Court), which implies they do not share spawn points. Furthermore, his spawn is much, much more infrequent than other rares who do share a spawn point with a normal mobs. I think it's far more likely that people got this impression because they just happened to be farming mobs in his spawn areas and were able to catch him easier than by just happening across him.
This is also one of the requirements for the Achievement Timeless Champion. He must have a low spawn rate.