junkyard dog

Short definition from Wiktionary

Noun

1. 1
   An aggressive dog which guards a scrapyard or junkyard.
2. 2
   An animal or person with an especially nasty and combative demeanor.
3. 3
   A hot dog laden with toppings, such as onions, pineapple, or chili.
4. 4
   Alternative form of junkyard dog.