One of the differences between the Druid teachings I follow and some other Western esoteric traditions is that Druidry does in fact recognize that souls can mess up badly enough to "flunk out" of human status and be reborn in animal forms. There's a Welsh tradition that someone with the second sight can see the cydfil or future animal form of someone who's headed that way, like a ghostly animal walking alongside them.

That said, the Druid teachings don't hold that souls have been wandering in samsara since beginningless time; while time has no beginning, swarms of souls have descended into material embodiment at intervals and worked their way up to what amounts to angelic status. We belong to the fourth such swarm in this part of this solar system, and no matter how badly we mess up, the pressures of karma drives each of us along the path toward Gwynfydd, "the luminous life," the condition of the awakened.

Who's right? Ask me that when we get there. ;-)