Signing your lease over to your roommate did not release you from liability to your landlord. Your landlord was not privy to your agreement with your roommate. You really need to consult with and possibly hire, an attorney who is experienced in landlord-tenant and collection matters. Do not contact the attorney without prior legal advice.
I agree. You signing the lease over does not change you contractual obligations with the landlord. Practically, if your roommate keeps paying under the lease, there should be no problem, but legally you are accountable for the money due under the lease. Lastly, I don’t know why an attorney would threaten you with sending a file to collection. Usually, it is the attorney that would take the matter to court, not send it out to a collection company that has no power to do anything. You might want to take all the written documents that you have pertaining to this rental unit, and see an attorney in your area. Good Luck.
Even if your former roommate (now) has a dog, if your name is on the lease you are both responsible for it until the lease ends or you have another agreement with the landlord. If she got a dog, a cat, a chinchilla or a bird you unfortunately will be responsible for her choices until you are OUT-OUT of the lease. Then, if you are charged in kind for bills or pet fees that are not yours you ask nicely for payment and when you don't get it (which you probably won't since she didn't pay the pet fee anyway) you file in Small Claims.