If you elected not to kill all the red dragons (much more likely you didn't even know about it), then you had to level up a new death knight if you wanted another attempt at it. While Unholy Determination did make it to live servers, it hadn't been intended.
Feedback from the community noted that a missable achievement was unfun for the players that missed it. While we've chosen to remove the Feats of Strength granted in the questlines overall, the voice line from Bolvar will still play if players do choose to go down this path as Death Knights.
As was alluded to, these Feats were not meant to go live, as we had previously decided that they were misaligned with our general look towards achievements, namely that they were unobtainable if missed in a non-repeatable quest area. Again, these will no longer be obtainable in 7.2.5, and for players who did obtain the achievement it will be removed.
This pet is available through the shaman class hall. You need to have done the quest chain for your first artifact to unlock it.
Once you have access, head to the Storm's Eye Overlook (labeled on your map) to find Snowfang. Bring out the Iron Starlette and use its abilities in the following order: Wind-Up, Supercharge, Wind-Up, and then Toxic Smoke. After defeating Snowfang, you will receive this item.