Roomate termination due to break of verbal agreement? (ATL)

Hello everyone. I am sorry I have to introduce myself on such negative terms, however I am in desperate need of your help.

My girlfriend and I have recently started renting an apartment together. Before moving in, we asked a friend if she would like to room with us. We interviewed this friend, and asked her some of the essential questions to determine if we could be possible roomates. These questions included things like:

Do you mind living with a couple.
Do you mind if I (Darold Cole) pay a slightly lower rent because I have another apartment, and will be providing a majority of the furniture/art/food/etc?
Are you ok with the fact that my girlfriend and I are planning on owning a dog within the next few months?

She said YES to all of these, plus a few other questions we asked to make sure that there were no major problems. After we knew that she was willing to live with these terms, we included her in the apartment searching process. This is when my girlfriend and I realized we may have made a mistake. She proved to be unwilling to compromise. Although we should have begun searching for a new roommate immediately, we were foolish, and continued to go out of our way to appease her. After getting into a stalemate about where to live, my girlfriend and I offered the ultimatum: We have decided on an apartment, and since we need to move in before school starts, we are going to sign this lease with or without you. She decided to join us in the lease (although she refused to sign ANY lease at first). Now the three of us our on the lease. Now she demands that I pay full rent, although we clearly agreed I pay a lower rent until I was finished with my pre-existing lease. I have bought over 80% of the furniture/art/food/other accessories. However, this is not the issue...

The other day, my girlfriend and I adopted a dog. When she came home and found this, she stated that she will not live with an animal, and quickly developed allergies (although she has two dogs at her home). She refuses to move out, and refuses to live with the dog. This animal is something my girlfriend and I have been planning on, and our views were clearly made visible to her. Had she told us that she would be unwilling to live with an animal, we would never have continued to consider her as a roommate. We allowed her to room with us as a favor.

What can I do legally? My girlfriend and I afraid that because we did not have a written agreement, we will be hurt by this situation. Although we could continue to pay for rent without the help of the roommate, it would be eextremelyhard for us to do. We would have to replace her as soon as possible. I also am aware that if the roommate refuses to pay us for the rent, my girlfriend and I are required to pay for her. Our apartment complex considers our apartment a single check, not three different people.

What can we do to protect ourselves? Can we force her to move out if we give her ample warning (2 months?). My girlfriend and I were planning on staying at this complex for a few more years (Although our current lease is for 12 months). We are only 3 months into that lease.

Any advice will be greatly appreciated. Where else can I go to for free/inexpensive legal advice?

We are afraid that we will have to continue paying for her part of the rent. If this happens, will we be forced to take her to small claims court? Is there any way that would allow us to either get her off the lease or allow us to break our lease without incurring penalty?