A Dinosaur is Here!? is an episode from the Doraemon 2005 anime. This episode is called Dinosaur Alert! in the U.S. English version/dub.
When live dinosaurs appear in the forest just outside of town, Doraemon and Noby go back a few days ago to solve the mystery of how they got there in the first place. In the end, it turns out Noby's irresponsibility with a series of gadgets is the cause of it.