This translation makes no sense to me. When would an English speaker say his mattress is in his bed in the same manner as his dog is? Or that his mattress is in his bed at all? Either this is not quite right or something is being lost in translation that I don't comprehend.
Tellingly, you're still writing that as "on the bed". I think seveer is right and the English translation should use "on" rather than "in". That would work for the mattress, the blanket, and the dog as well.