i can't remember the title of a movie i saw some years ago

A young boy finds a weird creature in his basement like a big lizard or a small dragon something like that. That creature is wise and has a human voice and he talks with the boy so I think he gives him some advise. The boy's father is at war and later the boy faces a dilemma about killing or abandoning the creature he found. He also has siblings and I think they live all together with their mother. Also, at some point the boy with his siblings write a letter, probably to their father, apologising for something.

I don't remember anything else about the plot but this movie is stuck in my head and I can't find it anywhere. So please if it rings a bell I would really appreciate some help.