Pterosaurs and plesiosaurs aren't true dinosaurs either, but they seem to be related to true dinos. Here's a better start:
Pterosaurs are distant kin of dinosaurs, both of them being Archosaurs, but plesiosaurs are no more relatives of dinosaurs than a snake.
Quakedancers are omnivorous beasts related to dinosaurs, though they are not true dinosaurs. They have developed an unusual means of hunting: they appear similar to herbivorous dinosaurs, but they have the ability to generate subsonic waves that can stun their prey. As the local fauna learns to recognize them as dangerous predators, they move to new hunting grounds, devastating one area after another.
Anything about reproduction and that legend?
Quakedancers are six-legged omnivorous beasts resembling long-necked herbivorous dinosaurs, though they are not true dinosaurs. They have an unusual means of catching prey: after rooting its central pair of feet in the ground, a quakedancer see-saws back and forth, stamping the ground with its front and hind feet so hard the resulting shock waves stun their prey, which it then swallows. This "quakedancing" has a devastating effect on the land, toppling trees and splitting open crevasses. It sometimes triggers worse cataclysms such as landslides, avalanches, or a proper earthquake. Humanoids who are familiar with quakedancers usually hunt them to death whenever they can, for the beast's earth-shattering power can ruin fields, break roads, or burst dams.
Quakedancers are peripatetic creatures. They move too slowly to chase most creatures, so can't catch prey that's learned to avoid entering their quakedancing range. Once that happens, the quakedancer moves on in search of new hunting grounds where the fauna does not fear them.
An adult quakedancer mates once a year, the females lay up to 20 eggs in a burrow at the center of a region the adults have devastated with their quakedancing. They then abandon the nest, leaving the young to raise themselves once they hatch. Quakedancers grow amazingly rapidly, reaching adulthood in five years.
There are no authenticated records of a quakedancer ever dying of old age, leading to rumors that they just continue to grow until they are killed by predator, disease, or disaster. These stories lead to the legend of the First Quaker, a mythical quakedancer of titanic size who is supposedly responsible for "natural" earthquakes.
