I think the word 'aggressive' is being used wrong. Terriers are... feisty, often times, to say the least. But 'aggressive' is not in the Yorkie breed standard.
As far CesarTheGreat and your pack dog theories. lol. This way of thinking is (now) so funny to me.
Not to mention, the beliefs that dominance trainers hold. Comparing anything that WE do with our dogs to how dogs react to other dogs is like comparing apples and oranges. We are humans. Dogs know this. We as humans can act like we're dogs, but we're not. We don't fool them. And just as humans are not dogs, dogs are NOT wolves. Dogs are highly domesticated, very intelligent, and purposely bred to work side-by-side with humans.
The actual pack leader/dominance theory phenomenon was disproved by the VERY people who created it. They now deeply regret their 'research'. (In a 2002 book, when countless dogs had been subjected to the stress of the alpha roll, the Monks of New Skete said that they regretted giving the advice to a bunch of dog owners with no idea how to use it.)
http://books.google.com/books?id=6OB...0alpha&f=false
I suggest these reads:
De-Bunking the "Alpha Dog" Theory - Whole Dog Journal Article http://avsabonline.org/uploads/posit..._statement.pdf TIME Magazine article on debunking Dominance/Alpha myth | Ruff Customers Dog Training & Behavior Consulting | Ruff Customers Dog Training & Behavior Consulting