Teldrassil: Sethir the Ancient does not move
http://wotlk.openwow.com/npc=6909
He is only static, even though he is supposed to walk up and down that branch.
However, all the pages and comments suggests otherwise, as well as some of the pictures posted online as help to this quest:
"Sahia on 2009/03/11 (Patch 3.0.9) I did this at level 10 as well. If you're careful enough to avoid the spiders or kill them you can wait for him to the entrance of the branch. Stay stealthed. When he starts walking away follow him slowly until you are within pick pocket range. Pick pocket and RUN remaining stealthed! I found this way to be easiest."
"Imf3ral on 2009/08/06 (Patch 3.2.0) This Helped Me, thanks. I did this with a level 10. I found that it's easy if you go up to behind where he spawns, infront of the giant branch (stealth) and wait for him to spawn. Simply then, follow until in pickpocket range and pick-pocket him. Then slowly run past him. "
https://c2.staticflickr.com/4/3040/2711466196_1250025f1e.jpg
http://farm4.static.flickr.com/3180/2711466176_6411a0ccff.jpg
https://c2.staticflickr.com/4/3242/2710654199_c956f6fb13.jpg