Tanpen Unico: Kuroi Kumo to Shiroi Hane

For one thing it was made by Sanrio in an era that transpire from the late 70s to mid 80s back before Hello Kitty cartoons and products became their main source of income, and they legitimately made anime. Another thing this has in common with Chirin no Suzu (maybe not to the extreme like the latter) is that it is relatively dark and the content might upset younger children.