We don't have evidence from scripture to say if animals go to heaven or hell. Rather, personally I think we should still pray for our pets because they pray (Psalm 150) in turn. In my opinion its divine mystery what'll happen to them. I think its up to God to decide, but I am optimistic to think God's love will extend to them as it does with us and angels.
C.S Lewis elaborates an interesting point here about it:
