People have all the attributes of animals, including senses, digestion, elimination, growth, corruptibility, health, etc. But there are two things people have that animals don't have and those are Intellect and Will.
By intellect is meant the ability to think, to contemplate cause and effect; to abstract, that is, to think of something that is not there at the moment. Being able to plan for the future and to learn from the mistakes of others. It also entails the ability or faculty of speech. There is no animal that can speak, although scientists are hard at work trying to find one, in their attempt to assert their rejection of religion. To be fair, it is not always the scientists, but the financial backers of their projects who are primarily responsible.
Speaking of speech, and scientists, they usually don't like to hear this sentence. "We do not think with our brain." I have seen scientists get tied up in knots trying to argue against that proposition. But it is quite simple to prove, to someone who has the eyes of faith.
The other thing, the will, is man's greatest asset, and his worst folly. It is by an act of will that we can put ourselves into eternal damnation or ascend to the heights of heaven, for mortal sin or heroic virtue is reducible to an act of the will.
Animals cannot think with intellectual acuity. They can only act according to their experience and instinct. They might seem to be making a "choice" in their behavior, but this is our interpretation of their animal thinking based on our desire to impose human faculties on animals that have none. Animals cannot make an act of the will, for they have no will. They behave according to what "feels right," and we ought to take note of this, for modern man seems to enjoy basing his decisions on feelings. He may not realize he is debasing his dignity of being human by doing so, which lowers him to the level of a brute animal.
Animals have a soul, but it's not a spiritual soul, for they lack intellect and will, and they're incapable of adoring God. That's why they don't belong at Mass! Don't bring you bird, snake or cat into the Church, please. When a person dies, their soul goes to its particular judgment, and then to one of 3 places, one of which is hell, and yes, Jennifer, hell is a place. When an animal dies, its soul returns to the potency of matter. End of story.
To think and/or believe that animals will be in heaven is approximately founded in a basic ignorance of what heaven is. Our experience is a prisoner of time, and everything we observe and consider doing involves time. A sentence is composed of a subject and a predicate, the latter of which needs a verb. Without a verb, you don't have a sentence, and any grammarian will tell you that a sentence is a complete thought expressed in intelligible language. Therefore, you cannot express a complete thought without a verb. But verbs are contingent on time, that is, without time verbs make no sense. The bottom line is, for us to think about a reality that is without time is practically impossible, for time is inherently essential for our minds to communicate with language.
But in heaven there is no time. So how can we think about heaven? How can we express our thoughts about something that has no time, and our thoughts must involve time for them to be communicated with intelligible language?
God created time, before he created anything else. The angels were created in time, and then after they were tested, they moved outside of time. But with God's permission they can step back into time, so to speak, however briefly. God exists outside of time, for God is eternal. And heaven is ultimately our sharing of God's eternal glory, which is something that does not involve time, and exists outside of time. This is a great mystery and is beyond our ability to comprehend, even though we enjoy the power of intellect and will. But in eternal life (or eternal death, which is hell) we will be perfectly cognizant of that reality and this is intimately related to the seeing of God face-to-face, which is heaven.
So how do animals fit into that picture? Remember, when we say "animals," we don't get to choose our favorite pet, and exclude that annoying neighbor's dog that barks all the time, because that dog is our neighbor's favorite pet and it's not our place to decide who goes to heaven along with their pet. Nor can we choose one animal species and not another, for some people are "cat" persons and others are "dog" persons, and still others are "horse" or "snake" or whatever persons. Nor can we exclude the fish in the sea, the birds of the air, the insects in the bushes or the microbes in between. We cannot exclude plankton.
If we add up the weight of all the insects that spiders consume worldwide in one day, it approximates the weight of the world's population of people. Add to that the amount of plankton in the seas, and you have an astronomical body mass. How does that fit in heaven? What purpose would it serve? We would have to allow all the insects, arachnids, plankton, paramecium, elephants, whales, tuna fish, sharks, gila monsters, antelope, porcupines, farm animals, and on and on and on, for the entire history of the world from the beginning of time. And for what?
Not a single one of them has an intellect or a will, and cannot contemplate the reality of God, and would have nothing to do, say or see outside of time.
Those who say they can't imagine a heaven without animals simply admit they can't imagine heaven. That's okay, because we really cannot imagine heaven, anyway. We will know it when we get there, and not before.
And anyone who dares to believe that "if heaven has no animals, then I don't want to go there," should be careful of what they ask for, because they just might get it.