Doraemon: Nobita and the Dragon Rider(1987)

Doraemon: Nobita and the Dragon Rider

Doraemon created a hideout for everyone in an underground cave and Suneo got lost. Everyone came back out except Suneo. After finding out that he was lost the entrance was broken. They have to find another entrance to the caves and end up in a dinosaur world. Vanho , a soldier rescued them and said that he found Suneo. When they found Suneo they found out about Vanho's people's plans. Their plan it to study Doraemon's group as human specimens and take over Earth's suface .

(Source: Anime News Network)