The red chest can generate a random content. Please notice that they can't be found exclusively in the new challenge rooms, they can also replace normal/golden chests. Until now I've found the following:
- spawn 2 red spiders
- teleport you to the Devil's Room/Angel Room
- give you a "Pretty Fly"-like halo of 3 flies, with the difference that each damage prevented may remove a fly from around your head. It stacks with Halo of Flies. Also the flies are of the aggressive kind (they'll chase enemies)
- the "Dead Cat" item, the one that gives you 9 lives.
- Guppy's Head, a usable item that gives you a halo of 2-4 attack flies, 1 room recharge
- Guppy's paw, a usable item that turns one of your normal heart containers in 3 soul hearts, instant recharge
- Guppy's tail, effect still unknown
- two pills
- two soul hearts
- two troll bombs
- single "super" troll bomb (the kind that follows you)
Another thing I've found: a "toothed" room, if connected to a secret room, will have another toothed door that brings you there. Be sure to have at least one bomb to break the secret room's walls, or you'll lose another whole heart to get out.