>Is there some sort of lore as to why sentinels and zealots (and maybe other types of DP I have not yet hunted) turn into stuffed burlap dummies when they die instead of like, regular corpses?

Look at their description:

Considered the master of disguise amongst the ranks of its fellow Dragon Priests, the sentinel prides itself in being prepared for any situation or setting it may find itself in. A dark silk cloak is draped from its wide shoulders to surround its body, partially obscuring a variety of tools and equipment utilized in the art of disguise and deception. Camouflage scales covered in an array of scars outline its well-toned frame, allowing the sentinel to blend in easily with most wilderness terrain. Just above its dark serpentine sneer burns the fire of hate in its eyes, fueled by ancient fanatical teachings of Dzree.

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Hunta Talna Kortok, built by Gor'Togs, for Gor'Togs
http://www.angelfire.com/rpg2/caraamon/home.html
Blunts for Sale:
http://www.elanthipedia.com/wiki/User:Caraamon#Wares
Combat Balance Sheet:
http://www.angelfire.com/rpg2/caraamon/balance2.xls