Like pego's they prefer to steal items in the last slot, so I have with success had a split stack of berries whenever I travel around these thieves; they'll steal 3 berries I care nothing about. Berries are dropped from dinos you kill as well, so I recommend just sticking a few on the bar if you see them around.
Me and a mate were walking home on dinoback with our two newly tamed pegomastx. We both had monkeys, so they had to walk. All of a sudden, a seagull swoops down and PICKS UP A PEGO. It started to fly away, but a slug from my shotgun turned it into Swiss cheese. Pilfer (the pego) was fine but traumatized. Never underestimate an icthy
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.