Dick King-Smith 
Guard Dog [EPUB ebook] 

There are six puppies in the pet shop window; five posh pedigree puppies, and a scruffy little mongrel with a grand ambition – to be a guard dog. The other pups laugh at him. How can such a small, scruffy dog possibly expect to be bought to guard a home? Especially when his bark is the most horrible, earsplitting racket they have ever heard! Will the poor little guard dog be doomed to a lonely life in the Dogs’ Home – or worse…?