The Monster Kid: Invitation to Monster Land (1981)

Kaibutsu Taro, also known as Kaibutsu-kun, the prince of Monster Land. Hiroshi-kun, a human boy who is close to him, takes advantage of winter vacation to head to Kaibutsu-kun's hometown, Monster Land. However, humans are strictly prohibited from visiting Monster Land, and Hiroshi-kun, who breaks the rules, is turned into stone by the power of the eyes of Kaibutsu-kun's father, the Monster King! Kaibutsu-kun goes to retrieve the secret medicine "Flower of Life" in order to restore Hiroshi-kun to his normal state. But that flower...