I didn’t understand. I don’t want stealing to be allowed because it would ruin society, and that situation would also harm me.
But why should I care about animals? If you say that it’s because harming them is immoral, then that’s just begging the question..
(It could be that I simply didn’t understand the categorical imperative correctly.)
And if the categorical imperative isn’t enough to say what a moral act is, then what is?

First, I’ve 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 acted this way or not (not necessarily from the standpoint of your own interest). Even if I’m the biggest bully in the universe, I still wouldn’t care if everyone were allowed to behave violently, because I’d overpower everyone. But that’s your own personal consideration, whereas the categorical imperative is built on the perspective of a typical person, not on you personally.

With intuitions I have no way to argue,
but as for the imperative—the consideration I presented is supposed to work for every typical rational person: permission to harm animals would not harm any typical person (let’s assume this is harm in a way that doesn’t damage the ecology); on the contrary, it would be beneficial (it’s healthy, tasty, and even cockfights can be very entertaining for certain people..). The only reason people would oppose such a general law is that they think it’s immoral—that is, they are begging the question (and here you’d have to bring in intuitions).

You’re assuming that when I say I wouldn’t want it to be a general law, I mean that in an interested, self-serving sense. That isn’t necessary.