Item Locking.[]
If you are really worried about security of your items/pets then you can safely lock them. Locking means that you will be able to used all those items/pets as before, but to trade/sell/free them will be more difficult.
Once enabled, it will ask you to set up a password after this you can choose which pet / item you want to lock, it will ask you to re-enter the password you just created.
You can then select the item/pet you wish to be locked. Once locked items/pets can not be sold or traded. If you want to unlock it you have to wait 3 days before it becomes available to sell/trade again.
You can access it from your bag by clicking Lock button on the left land side button at the bottom of your rucksack. You will see the locking menu with different tabs: items, stuff, pet.
Lock/unlock pet function also available from pet screen (Alt+X).
Additional Notes:
I noticed that once I had locked my pet, I went to my bank and it asked me to enter my password (that I created for locking) to be able to access my bank. Which means that Lock function activates security password and once activated, you will not be able to trade/buy/sell/access your bank safe until you will enter this password.