We have a good array of other creatures, from indomitable apes to baby stealing foxes. There are two things I think we lack in this department; any reptilian entity, and threats that could outsmart our nichelings. We have the stealth, scentless, and camouflage characteristics, and you can even see them in action with wandering creatures, but right now we scarcely have to worry about them being used against us. These facts in mind, I think that we should have some form of stealth predator, something which could harm our creatures and escape with minimal fear of retribution.
Since snakes and lizards alike can be stealthy, I propose a combination creature of some form be made from them. To further emphasize the threat of this creature, I think that it would be poisonous- not especially likely to kill a given nicheling, but threatening none the less. Not only would this continue to turn tools like poison against us, but it would make some traits more useful; on some islands-including the Burning Savannah, a probable habitat for this creature- toxic berry bushes never show up, making poison resistance pointless. The Rattle Lizard- or any other poison opponent- would change that, making the resistance much more likely to save a creature's life. This would be especially true if our tribes were on an island without the healing fruit
Lastly, I would like to suggest stats for any stealthy assassin creature, Rattle Lizard or not: 1 strength, 3 or 4 speed, 2 stealth, 2 scentless, 1 camouflage, and the poisonous ability.
*Addition* I got a little bit enthralled by this idea, so I decided to make some art for this creature...with a program I've literally never used before.