I get stealing and eating meat and berries, but CRYSTAL JARS AND CANTEENS!? How does it not immediately die trying to swallow that?

When you think nothing could be worse, and there you have it, a flying pegomastax!!

Similar to the pego, these guys are also assholes. Especially when you're taming and they steal food out of your tames inventory and reset the tame process. Major assholes.

It'll steal your sacrifices and give you fish in return!

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.