Dueling Movies: An unintentional example: the movie was released the same year in Hungary when Disney released The Fox and the Hound in the USA. Both movies star a fox, who is pursued by a hunter and his dogs. Since very little information went through the Iron Curtain about animated films in production, it is unlikely that the creators of either movie heard of the other project.
Outlived Its Creator: Yes. The Sequel. But this began with István Fekete having a son (also called István Fekete) who though that he absolutely must write sequels for his father's work (he owned the rights after his father's death anyway)... unfortunately he neither has the talent nor the understanding of biology. The one for Vuk got licensed and the rest is history.
What Could Have Been: The original makers actually intended to produce a sequel for this movie. A traditionally animated one that kept the original character designs (except fixing the Humanlike Foot Anatomy) and focused on the adventures of a white fox with no input from István Fekete junior.
Write What You Know: István Fekete (the older) was born in a village, had a degree in agriculture, and was an active hunter since childhood. Let's just say it shows. (For comparison István Fekete junior was/had neither and that shows too.)