According to Osho, a human can certainly have been an animal (even a bird, insect, or stone) in past lives, because existence evolves upward in consciousness. But once human, you cannot go back to an animal birth—there is no return, only progression or stagnation. Earlier layers persist within us, which is why under stress our behavior can appear animalistic; the path is to grow awareness.
Yes—you may have been an animal before, but after becoming human you won’t be born as one again; you either grow in awareness or stay stuck, though old animal tendencies can still pop out.