A threshadon (a.k.a. thresher) is a large, long-necked plesiosaur that can only swim. They are found in the watery areas of Darkshore, Wailing Caverns, Loch Modan, Lake Everstill, and other areas.
A threshadon boss named Nessy can be found in the underwater section of the Deeprun Tram. Apparently, the treshadon produces ambergris (see Threshadon Related Item section below).
In the Blackfathom Deeps, there is a threshadon mini-boss named Old Serra'kis that you can defeat. Also, an elite threshadon prowls the waters of Lake Everstill in Redridge Mountains.
Note that Bloodpetal Threshers and Root Threshers are lashers, not threshadons.
The Threshadon is similiar to the Plesiosaurs such as the muraenosaurus.