Some of the animals Bosch painted are very realistic, especially the birds that he could observe around his home. Others, like the giraffe and the leopards he copied out of books.
Take a look at this detail from the left panel; the two birds on the left look realistic as are the frogs hopping around (and getting eaten). Even the seal is fairly life-like. But we can also see some truly weird animals; a flying-fish, a swimming unicorn, a three-headed bird, something resembling a swimming rooster and – perhaps strangest of all- a duck reading a book.
This is one of the many unexplainable aspects of this painting; why did Bosch paint this strange mixture of real and imaginary animals and what was he trying to tell his viewers? It is tempting to think that it was easier for his contemporaries to understand, but it was not. Ever since it was painted people weren’t sure what its message was and centuries of research and discussions have clarified little.