It’s an interesting debate, but probably not one for these forums.
Collins English dictionary gives this as first definition.
An animal is a living
creature such as a
dog,
lion, or
rabbit, rather than a bird, fish, insect, or human being.
We are already in trouble. Why aren’t humans listed as animals? You can see where this argument leads. Not a debate for this forum certainly.
If one kept only fish and in casual conversation mentioned to someone you had to go home to feed the animals you may get some odd looks and a comment such as I thought you only kept fish.
On the Ministry Of Agriculture forms here you are asked what animals you keep. Bees and fowl, which we keep are not listed in the animal section. So, here and in many other places, insects are not generally classified as animals. You would not should you only keep bees, say you had to return home to feed the animals.
So, at best, animals is then left as a very loose description of all living creatures except strangely humans. Back to that tricky subject again.
So, my view is, given what we know about evolution, we either include all creatures as animals, including humans, or we use another word, or be species specific.
It gets more complicated. Our classification of species is still largely Darwinian. No doubt he was a clever man but we have, or at least science has, moved on and our knowledge increased.
Chickens get referred to as birds. But, if the theory is correct and chickens were in fact a type of dinosaur which had small arms that later developed into wings and given most would acknowledge chickens have never flown like most other creatures we classify as birds, then it’s hard to see how the classification as bird is reasonable.
So, if chickens are birds then animal doesn’t seem correct. If they were dinosaurs, then they are animals and not birds.
Here in Spain and in the UK chickens are classified as fowl. Fowl is used to distinguish a number of species from birds. Perhaps this is the best compromise.