The special Siesbarg mount comes from a long quest chain which requires a great deal of farming, though players can make that grind significantly easier by chain running low-effort follower dungeons!
Siesbarg is the final reward of Consolation Flies, a six quest chain beginning with the Vial of Tka'ktath's Blood looted from the powerful Tka'ktath rare-elite within Azj-Kahet. Upon turning in the blood, which also requires Renown level 4 with the The Severed Threads, players are set to the task of acquiring several thousand materials in succession. The quests aren't concurrent either, so while you could just keep them in your log and complete each slowly over time, it's likely to take quite a while.
This is the hard part, as farming these materials can take several hours, even in a group scouring an entire area's worth of mobs. While there are several viable spots, including Faerin's Advance and the Siegehold in Azj-Kahet, or the Tranquil Strand in the Isle of Dorn, they also drop in Ara-Kara and the City of Threads dungeons without having to group up or contend with other players.
He's kind of hard to miss, but has a lengthy ~7 hour respawn timer.
This presents a unique opportunity, as both of these dungeons are eligible to be run with followers — AI controlled companions who will help support you throughout a normal difficulty dungeon, perfect for chain running a few (dozen) instances and claiming all the drops for yourself!
This is by no means a fast grind, but it is easy and doesn't require any other players, so turn on your favorite TV