Woo! You've given so much more ethos to Nagual than I ever did. This looks awesome.
Specific comments below.
Whenever a target is destroyed place it beneath this card. At the start of the Environment Turn if there are (H)x3 beneath this card. GAME OVER. The heroes lose the game.
I'd suggest adding some sort of mechanic to get rid of this card. Otherwise, players will lose if they don't have an Environment destruction card, and immediately destroy it if they do.
Berserk Native 6HP
When this card enters play it deals each target 1 Melee Damage. One hero may deal themselves 2 Melee Damage to flip this card.
I like the flip mechanic, but you need to specify when this happens. Is it also when the card enters play?
Also, this doesn't seem to do anything once it's out.
Stealthy Blowgunner 3HP
At the end of the environment turn this card deals the 3 Non-environment targets with the lowest HP 1 Toxic Damage. One hero may skip their play phase to flip this card.
Greedy Miners 3 HP
The hero with the highest HP cannot deal damage while this card is in play. One player may skip their draw phase to flip this card.
This one is really nice. Dangerous effect I haven't seen anywhere before. My only concern is that skipping a draw phase is not much of a price -- they may disappear too quickly.
Maybe the Blowgunner could flip on the skipped draw phase and the Miners could flip on a skipped play phase?
Fleeing Adventurer 4HP
No Hero cards can be played. At the end of the environment turn if (H) cards are discarded flip this card.
I think you need to note "At the end of the environment turn, any player may discard any number of cards. If (H) cards are discarded, flip this card.
Deadly Tree Frogs
Each non-environment target in play takes 1 Toxic Damage. Any Hero that takes damage this way must discard a card.
You need to specify that the effect happens when the card comes into play, and probably that the card is destroyed at the end of the environment turn.
Those #&*!@ Monkeys!
Any hero with equipment in play must discard 1 equipment card.
Likewise on this one.
Heroes cannot play cards or use Powers while this card is in play. At the end of the environment turn one player may discard their hand to destroy this card.
Suggestion: This one is sort of a drag right now. Someone has to discard their hand, because it's so harsh. But discarding your hand typically puts one player out of the action for a few turns.
As-is, it might as well read "One player must discard their hand."
Suggest "discard all but one card" instead.