Dropped from Arcanist Doan in the Scarlet Monastery Library instance. It was not a part of the original table, but was added in Patch 2.3.
This ring no longer exists in game.
Prior to the dungeon's revamp in Mists of Pandaria, it was a rare drop in Scarlet Monastery.
Although it was a great tank ring, it was also a phenomenal and highly-sought item for battleground twinks in the level 29 bracket. In addition to the awesome stamina, the Freeze equip could even proc off of pets for pet classes.
The proc does not work at the level cap nowadays. I think, but am not sure, that it will not proc on anyone above level 60.
If I understand the Wowhead comments, the proc effect scales to level - the higher the character's level, the lower the proc chance. At current level cap, I imagine the proc chance would be so small as to be practically non-existent. I don't think I have seen Blizzard mention the item in any hotfixes, so Wowhead comments are probably correct.