But it did appear as OGC in Fantasy Flight Games' Draconic Lore, of which I have a copy. As written, it was worded for dragons, but the version in Stormwrack simply re-wrote it for monsters in general.
If we didn't just stick to core stuff... It would be a good substitute feat for someone to give this, though. You know, I wonder if this is just convergent design or if WotC is just ignoring their OGL obligations.
Many jokingly call the mythical lake monster "Esmelda." Many stories describe the monster in similar ways, though inconsistencies creep in: "Body longer than two boats"; "long neck like a snake, but a head with jaws like I hear dragons have"; "it had a ridge of plates or spines running full-length along its neck and back"; "its flukes and flippers were the size of our pleasure craft"; "its entire body was like a sea snake, and it could've wrapped the whole boat up and crushed it"; "its eyes were intelligent and it cast a paralysis beam on us that froze us right there"; "it seemed playful, coming up just beneath the boat, popping its head out of the water to eat the net with my day's catch in it." A few folk around the lake swear to have seen this creature, claiming it must be a dragon given its reptilian features and massive size.
"Esmelda" is all too real--but she is not just one monster. The depths of the lake support over a dozen cold/loving waterbreathing reptiles called pythosaurs, which come to the surface only if provoked or hungry. A pythosaur will attack boats as well as sailors.
Pythosaurus (1; 13): AC 4; MV 30; HD 18; hp 72 (ave.); THAC0 15; #AT 1; Dmg 5-30; SA swallow; SD none; SW none; MR none; SZ G (body 120 feet, neck 70 feet); ML Elite (13); Int Animal (1); AL N; XP 7,000, Notes: SA--When attacking any creature under 10 feet tall (L-size), this creature swallows the victim whole on a THAC0 of 19 or 20. Swallowed victims suffer 2hit points damage per round until dead.