One dog and his boy - Eva Ibbotson

But a dog would damage the expensive carpets in Hal's glamorous home,
and his wealthy parents refuse to consider one. Then they discover
Easy Pets, a convenient dog-rental agency. Terrier Fleck arrives
on Hal's birthday, but when Hal discovers that his dog must
be returned, he runs away. Hal and Fleck are joined by a group
of pedigree breeds joyfully escaping from Easy Pets - among them
is Otto, the wise and sombre St Bernard, and the fierce and excitable
Pekinese Li-Chee. A large reward is offered for the missing
boy, and soon Hal and his dogs are being chased across the country
by ruthless pursuers.
Helped by a travelling circus, and the sympathetic
children from an orphanage, they race for their freedom
in a classic adventure in the tradition of 101 DALMATIANS.
Every
dog-loving child will adore this irresistible story from a truly
great and much-loved children's author.