Split large stacking consumables such as narcotics a couple times as they steal full stacks at a time. Also they can even steal your water containers (canteens) and you can’t get them back. I recommend you stay away if possible or shoot them with the shotgun or bring dimorphodons to swarm them from a distance
I was attacked by one while mining flint and it stole my pick. Then it got my pike when I went to fight. That's when I noticed noticed that it didn't actually "steal" what was in my hand, but just disarmed me and dropped it in the ground. Check the ground and see if your stuff is there after they attack you. My pick was laying there near the spot of the initial attack.