In former times people believed that animals do not have a soul. Now you rather ask the question: What makes us think someone has a soul?
So if you have emotions, if you can suffer, if you can love- you have a soul.
And of course there are animals which are very close to us in these terms: dolphins, dogs, cats etc. Most of us cannot see these characteristics as easily in a spider or a fish but most humans would accept it in many other animals.
And now if you agree (and I would) that cats have a soul- why should god make a difference. I do not think he will.
If there is heaven, then there will be kitties there.
While I agree that the Rainbow Bridge legend may not be exactly right (anyway, noone can proof it is wrong) I am aboslutely sure that our pets will be where we are.