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).
Isha Awak, the Deep Doom, is a green-black threshadon feared by people at the Merchant Coast of the Barrens (according to Mahren Skyseer, he has eaten many humans).
In the Blackfathom Deeps, there is a threshadon mini-boss named Old Serra'kis that can be defeated. Also, an elite threshadon prowls the waters of Lake Everstill in Redridge Mountains.
Note that Bloodpetal Threshers and Root Threshers are lashers, not threshadons. Also, Fenclaw Thrashers are hydras and Highland Trashers are raptors, while Redridge Thrashers are humanoid Gnolls.
The Threshadon is similiar to the Plesiosaurs such as the muraenosaurus.