Być with to, ta, and ten.
Do nouns not take the instrumental case when dealing with to, ta, and ten? For example Ten jest kot; why isn't it 'ten jest kotem'?
you are mixing up two usages of this.
"ten jest kotem"= "this one is a cat" is a correct sentence.
But you wanted to translate "this is a cat"=to jest kot, and in sentences like that, you only use "TO", regardless of the gender and number of the word. And you use nominative in sentences with this "to". You can also omit jest/są in those sentences.