The Sylvan Stag is an old mount, added to the game way back during the Sharandar campaign, but it was the first and only lockbox mount that I recall ever actually wanting. Not enough to actually take part in Neverwinter's lockbox shenanigans, mind you - and the price on the auction house was always prohibitively high for a casual player like me.
The recent appearance of these stags in Nostura's Forest reminded me to have another look to check the current rate they were going for, and lo and behold, they were only a fraction of what they used to be! Still by no means cheap, but affordable even to me, so I finally gave in and purchased one. I decided to give it to my hunter ranger since that seemed most thematically appropriate, and my cleric main has enough other mounts that suit her anyway.
The only thing that remains mysterious to me is why the price has dropped so much. A (very) cursory Google search seemed to indicate that a lot of previously rare mounts and companions have been added as (also rare) reward drops to dungeon and skirmish chests, which would certainly explain it. Good times for me in any case.