"As an additional cost to recruit Mammoth, reveal another Fearsome Five character card from your hand.
When Mammoth comes into play, KO a resource you control.

Whenever Mammoth becomes stunned, KO him.

When Mammoth is put into a KO'd pile from play, put the top card of your deck face down into your resource row."

If Mammoth comes into play stunned, does his effect to KO a resource trigger? I would think no, since his text should be inactive as he is stunned. I know the last part, where he's KO'd and puts the top card of your deck into your resource row will work while he's stunned, since that's the way he's going to be put into the KO pile. I just didn't know if Mammoth would have to be recovered at some point for the "When Mammoth is put into a KO'd pile from play" portion to become active. I can see where UDE would rule that coming into play effects will happen even when stunned, just to try and avoid non-sense like only the resource addition of his effect happening, but I'm a non-sense fan.

Mammoth received an errata to make sure that doesn't happen. Normally, a character entering play stunned would not trigger his/her own come-into-play power.

EDIT: To clarify, now both the resource KO and the resource replacement both only happen if Mammoth comes into play un-stunned. When the CIP effect resolves, it KOs a resource, and then creates a triggered modifier that puts the top card of your deck face down into your resource row when he next enters a KO'd pile from play.

Alright, so at least I understood the rules correctly, he was just errata'd. (Back to the drawing board on breaking Underworld I guess)

Is there a list of all erratas?

Where are the erratas posted?

I dunno. Shimmer + all the swarmy guys that haven't been KOed the entire game still on your board (teamed up with Fearsome Five, natch) would still be pretty brutal on your off-initiative kill turn.

If you want to check for an errata, just go here and check the Official Card Reference. That document shows the current legal game text for every card released.