The Indian in the Cupboard, by Lynne Reid Banks

This is a children's/young adult fantasy novel. An interesting premise: a young boy is given a plastic Indian (native American) figure, the kind of toy that comes in a package of many. There are many kinds of these sets: cowboys and Indians, soldiers, farm animals. Not very grateful for the toy, he stashes it away in a "cupboard" (medicine cabinet to Americans; Ms. Banks is a British author) and locks it with a very unusual old-fashioned key that his mother's grandmother had given to her long ago. Next morning, he awakens, hearing mysterious noises in the cupboard which he is keeping on his nightstand.

The Indian, though still very tiny, is alive! Alive, and not at all happy to be snatched from his Iroquois people of 1700s North America. Gradually, the boy, Omri, and the Indian, Little Bear, begin to understand and respect one another. As one might imagine, it is of utmost importance to keep Little Bear a secret from Omri's parents and brothers and his best friend Patrick, who gave him the plastic Indian. As Little Bear understandably becomes more and more insistent on having his needs met, Omri feels a deepening sense of responsibility that begins to weigh on him heavily. As he thinks of the future, he realizes that decisions must be made, and soon.

This book was made into a movie, and several sequels were written.