The dinosaur, an Apatosaurus named "Tremor" is the unofficial third crew member (in the DM2 webcast also sometimes jokingly referred to as a stowaway) of SpX-DM2. It was chosen by Doug Hurley's and Bob Behnken's sons, who are both dinosaur fans.
Plush toys have somewhat of a tradition of being used as a zero-G indicator in spacecraft, since they are small, lightweight, fun, and pose no danger to equipment or crew when floating around.
This is not the first dinosaur on the ISS. In fact, there was even a dinosaur born on the ISS: Karen Nyberg sewed one for her son, who is, of course, also Doug Hurley's son.