Sigmund and the Sea Monsters (1973)

Sigmund is a sea monster. He's also a tremendous embarrassment to his family because, unlike a normal sea monster, Sigmund has no desire to scare anybody. He runs away from home rather than scare people, and meets up with Johnny and Scott. The three become best friends and Johnny and Scott keep Sigmund in their clubhouse. Johnny and Scott spend their time alternately (A) keeping Sigmund from being kidnapped by his family and (B) keeping him a secret from everyone else.