She didn't sign a lease that said she couldn't have rabbits or had to stay if she felt like leaving. Another person didn't sign anything so legally tomorrow that person can say she is moving out to live with her boyfriend.

Life lesson, get everything on paper and signed to protect your own tail.

"The only thing is only one of us signed the lease"

Whoever that is, is legally STUCK even if all the others move out. Keep her "deposit", and consider this ot be lessons learned. GET WRITTEN LEASES from everyone.

"A lease can be a oral agreement and we could prove that she had agreed to the lease."
You could try to take her to small claims court for the money, but at best you get a court order, not money.

When she paid her share of the rent, was it by check, did it state in the memo slot that is was for her rent? Do you have anything, even e-mail that said that she was willing to share responsibility? Has she made her requests for deposit in writting? (and if she did, did she say anything about breaking her contract with you?)

I agree with the others, you are sort of stuck. Keep the deposit, go to small claims, especially if you have anything other than oral promises to pay rent for the year.

Susan

One thing you didn't mention and I think it's important, did she have the rabbits when she moved in? Or did she bring them in after she moved in, without consulting anyone?

Without a written lease she would be considered a month-to-month tenant, meaning she can leave at any time. Unless she signed a 1 year lease stating no pets, then honestly, you're out of luck.

For now, perhaps you should be putting all your efforts into recruiting a replacement roomie as soon as possible--you may never get another dollar from the rabbit-lover and you need to find someone to make up the rent shortfall.