Emile is one of the earliest children's books written by Tomi Ungerer. It was published in 1960. Ungerer based several books on animals that were looked down upon and an Octopus was not a very popular creature in children's books back then.
Ungerer wanted to write stories about animals that were despised and display the fact that everybody has got something special that the others dont have, so why not show their difference and make them special.
You can read more about Ungerer here- https://talesfortadpoles.ie/blogs/news/tomi-blog