I don't think it would be in any way a deal breaker ( or a covenant breaker?) if some unfallen rational animal were discovered somewhere, but I think the easier interpretation is that the whole shebang (theological term) was cursed when man fell.
There are already fallen rational creatures that are not going to be redeemed. They are called demons, the rulers, the authorities, the cosmic powers over this present darkness, the spiritual forces of evil in the heavenly places (Ephesians 6:12)
Redemption is for mankind and that's all we need to concern ourselves with.
All these *what if* mental exercises about the possibilities of something that is not likely to ever happen are an exercise in futility.
Christians would be better off worrying about how to deal with a lost and dying mankind, to whom salvation is offered, than hypothetical scenarios about beings which may never exist.