Actually, I wouldn't put it past them to use oak for a box like that. Especially if they are trying to keep something strong it in, like a monster. Yes, spruce or pine is lighter and softer, and therefore easier to work with.....however oak gives you the overall strength of the wood as well as the heaviness of it for big projects, like building a box for the sole reason of keeping a big, nasty monster in it. Knowledge of this? I have been a carpenter for 10-11 years now