Three from Prostokvashino

Uncle Fyodor is a very independent city boy, "a boy on his own". After his mother forbids him from keeping his talking cat Matroskin, Uncle Fyodor runs away from home to live on his own. He and his friends arrive at the village Prostokvashino, where a house is provided for them. There is a lot of extra space in the house, and therefore the local dog Sharik was called to fill it; he joined them cheerfully and amicably.

Uncle Fyodor's parents became very agitated at the loss of their son, and even put out a missing persons notice in the paper… Such a notice couldn't pass the nose of the extremely curious postman Pechkin, who right then and there declared his hopes to earn a reward for the boy's safe return — a new bicycle. Read more on Wikipedia.