One day the boy found a strange creature. It could not belong the real world, so he tried to find where such creatures live. He finally thought about it less and less, or maybe just stopped noticing them.
I think the lost thing is the symbol of person's individuality. These days people are losing their own individualities and try to behave in the same way as others, and they hate other people's outstanding personalities. The movie tells us the importance of getting one's own individuality.
