What animal was the symbol for liberty in ancient rome?

They had several first the Eagle held at the top of each army group's banner it was considered the greatest dishonor to lose one of Rome's Eagle's. Also the Chi-Rho after the change of the Roman Empire's officail religion to Christianity it is made up of two Greek letters: Chi and Rho respectivly super imposed over each (the two letters are the first two in the name of Christ).