We have video footage of death threats, proof from department of public works, plumbers, board of health, and other contractors backing every single thing up I have stated.We of course will screen better next time, but first we need to get her out, which seems impossible right now.

The simple answer, I imagine, is to serve them a notice to comply and require the additional deposit or the removal of the pet. however they are the best tenants I've had in my investment career (4 years), moreover evictions are temporarily on hold till who knows when so my threat to evict can be drawn out dramatically longer.

Lots of people use it as a threat but often don't follow through. 

The threat of a big suit will hopefully help you get to where you want to be.

If the Buyer indicates they will hire a Buyers Agent, I would prefer they be represented, I would not view it as a threat at all.  

It took my threats of canceling the contract, our escrow officer threatening to resign, and our loan officer going to his manager before BofA's funder finally got it together.Oh, sorry, this was supposed to be the Official Chase Bank Sucks thread.

He kept making threats every time he wanted something.

If the owner has proof that the tenant was notified prior to their lease signing and it doesn't impact their use of the space then I'd disregard their threats. 

Time for your attorney to send out a threat letter.

Next, don't listen to @Bryce Wong, you're obviously not a troll, in fact his mentioning this makes him libel, you could easily sue him for that and possibly argue that he was cyber bullying you.