It's resource guarding, if you put that in the search bar you'll find tons of threads on the subject
For now, take away all toys and treats while they're together. It's just going to cause stress for both dog and human until it's sorted. Dogs don't understand the concept of sharing as humans do, they just know the dog is taking something of theirs away from them and they have to defend their right to own it.
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010