Kappa: A River Goblin and Sampei 1993 カッパの三平

Adapted from Shigeru Mizuki's classic manga from the 1960s, 'Kappa no Sanpei', the anime tells a story of a young boy, Sanpei, who lives in the beautiful countryside with his grandfather. One day, he is caught in the muddy streams of the river and wakes up in the land of goblins. Sanpei makes friends with a goblin named Gartaro, who bears striking resemblance to himself. Thus, the adventure begins.