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.
Common around the coast. They WILL steal food from you and the creature you're taming. Kill them by bola-ing them and either shooting them or hitting them, they have low health so it's not too hard to quickly get rid of them.
These oceantic avian dinosaurs (birds) will steal any snacks you have on you except for kibble and will do so viciously but when tamed can do the same and make the flesh of small animals (salmon, celocanth, dodo, compy, pegomastax, lythrosaur, trilobite) tender and tasty (there prime)