Der 35. Mai (The 35th of May, or Conrad's Ride to the South Seas) is another children's book by Erich Kästner of Emil und die Detektive and the original The Parent Trap fame.
Konrad is a boy who has a problem: As his homework, he has to write a story about the Southern Sea, and he's lacking imagination (well, his teacher says so). But he and his uncle (whom he visits that day) get help from the talking circus horse Negro Caballo. They go through the uncle's wardrobe to the magical lands -- if this sounds familiar to you, yes, this story inspired the Plot Device of C. S. Lewis' The Lion, the Witch and the Wardrobe.
There's an Opera of it, made in 1931.
In the prologue to Emil and the Detectives, Kästner wrote that he wrote the story about Emil because he rather wanted to write a story like this, but couldn't remember how many legs a whale-fish had.