While I don't know much about this (I can hear all those nods of agreement),
another particular may be relevant. In the time of David I a "great worm" was
reported as doing great depredation in the area around Linton. A descendant of a Norman knight who fought at Hastings went to see, and devised a lance
with a small wheel near the tip to allow the tip to go down the beast's throat,
it worked, and the Somervilles were given an entire shire. So the story goes.
Their arms and crest show a knight's helmet surmounted by a dragon perched
on a wheel. Some apparent fact, some possible fancy; voila! A mythical real
heraldic beast.