It can also drop from the Shadowlands Pre-patch event which will spawn Skai the Ruthless every 6 hoursish. It is assumed to be on the same drop chance as dungeon version.
The entrance to Utgarde Pinnacle can be found on the North side of Utgarde Keep, Howling Fjord. The mount can only drop on Heroic, so do ensure you are on Heroic Dungeon difficulty before you enter by right clicking on your character portrait and changing the Dungeon Difficulty.
Skadi the Ruthless is the 3rd boss of the dungeon but you don’t actually have to kill the first and second boss, they can be simply walked past as you make your way to the boss of interest.
Once you reach Skadi the Ruthless, you need to follow him down the pathway, defeating the adds in the way and then continue to kill the adds. The adds named Ymjirjar Harpooner will drop harpoons. Proceed to collect three harpoons and then use them on the Harpoon Guns but only when Skadi and his Proto-Drake are in front of the Harpoon Guns.
If you successfully hit Skadi with three harpoons then he will land, allowing you to kill him and loot him for a chance at the Blue Proto-Drake.
Extra Tips: Since you can only clear a heroic dungeon once per day per character, run the dungeon on your alts to have more than one chance daily.