Harm to animals

I don't understand. I don't want it to be allowed to steal because it would destroy society, and such a situation would harm me too.
But why should I care about animals? If you say that because harming them is immoral, then that's already the desired assumption..
(I may not have understood the categorical imperative well at all)
And if the categorical imperative is not enough to say what is a moral act, then what is?

First, I have written more than once that the categorical imperative is one component and not the whole of morality. Beyond it, there are moral intuitions that are valid in themselves regardless of the imperative.
Beyond that, the question of whether you care or not is a personal question. The categorical imperative can be formulated in an objective way: would the world be better if everyone behaved this way or not (not necessarily in terms of your interest). Even if I am the biggest bully in the universe, I don't care if everyone is allowed to behave violently, because I will overcome everyone. But this is your personal consideration, and the categorical imperative is based on the consideration of a typical person and not yours personally.

I have no way of arguing with intuitions,
but regarding the decree – the consideration I presented should work for any typical rational person: allowing harm to animals would not harm any typical person (let's say it is harm in a way that is not harmful to the ecology), quite the opposite (it is healthy, tasty, and cockfighting can also be very interesting to certain people..). The only reason people would oppose such a general law is that they think it is immoral, meaning they assume what is wanted (here you will have to insert intuitions).

You assume that what I wouldn't want to be a general rule is self-interested intent. It's not necessary.