Cornelius

Cornelius is a small, white creature owned as a pet and familiar by Matilda Pilqvist in the city of Trolberg.

It's an oddly white furry domesticated animal with tiny buck teeth. It first appears within the book "Hilda and the Bird Parade" and then the animated seies in "Chapter 3: The Bird Parade", where it had the same role in both. The creature is given more focus in "Chapter 3: The Witch". This episode reveals Cornelius is Matilda's familiar. He is able to swallow objects many times his own size, and keep them inside his body for an undetermined amount of time.

Trivia

 * Cornelius also made a brief cameo appearance along with Matilda in "Hilda and the Black Hound".