Legendary Bear: Died during fight, can't find him anymore
Same thing happened to me, what I did was ride a decent bit away and set up camp. Slept until the next morning then rode back up towards the icon. When I did that a notification popped up in the top-left part of my screen saying I was in the a legendary animal's territory. Seems like if that notification doesn't pop up it won't spawn the bear so make sure it does before you spend any more time looking for it. Took me awhile to figure that out, hope it helps.