Hobbits were no part of Tolkien’s original plan. They entered rather late and through a side door, as the unexpected central characters in a children’s story, The Hobbit which Tolkien invented for his own children but which found an immediate and lasting worldwide audience. And of which The Lord of the Rings was the commissioned, longed-awaited and trebly successful sequel.

The Literary Power of Hobbits: How JRR Tolkien Shaped Modern Fantasy is a good short read (via)