I like the idea, if you went down the Life route you could call the upgrade Fox Hole maybe.
Just a question, you say temporarily in the original description does that mean they come out after X turns or do you have to reveal them yourself.
On another thought, can you hide them and then your opponent can't see what is in the Den? If this was the case you could play Den this turn, on your next turn play 3 Dragons and Hide them (don't know why but it would really mess with the pvp opponents head wondering where the dragons are) and then boost them while in hiding. Unsure how that would work but an idea.
Good work and nice idea.
Scyther