I'm guessing the term refers to
Seismosaurus, an exceptionally large sauropod originally estimated to have been 150–170 feet long. Its name literally translates to “earthquake reptile”. A recent reevaluation of the
Seismosaurus fossil material concluded that its size had been miscalculated; the revised estimate is 110 feet. Furthermore, the specimen is now widely considered to represent a large
Diplodocus (the same species as Doc from LBT VI), so
Seismosaurus is not really considered to be its own dinosaur anymore (much as
Brontosaurus is now considered to be a species of
Apatosaurus). Still, scientific accuracy has never been a major concern with LBT,
and the term could apply to other large longneck species such as
Supersaurus (Sue from LBT X).
By the way, can you please tell me what fanfiction you are talking about specifically? Just so I know what your source was. Thanks for your submission.