How would u say the following?

Hi,
I would say:

This is him, isn't he?
These are them, aren't they?

This is a cat, isn't it?
This is him, isn't it? / He's the one, isn't he?
This is it, isn't it? / This is the one, isn't it?
This is them, isn't it? / They're the ones, aren't they?
These are the ones, aren't they?

CJ

Whoa! I really didn't know that! You always say "isn't it"... and "this is" even if the pronoun is plural.

This is them, isn't it?

Is it really odd to say "This is him, isn't he?" Thanks

These endings come very natural to me as a native English speaker, but they must be **** for those who are learning English. I spit out "...haven't they?", "...aren't we"?, "...doesn't he?" effortlessly.

In the sentence you provide:

"This is them, isn't it them?"

I assume that "this" (the subject of the sentence) refers to a group of people. "This" is singular,

How would you say the following, not u.

Is it really odd to say "This is him, isn't he?"

Yes!

Come on, Kooyeen!

The tag has to question the subject, not the complement.
Within the tag the subject is pronomialized to become he, she, it, or they. The pronomial form of this or that is it. The pronomial form of these or those is

Ah, ok, I see. Thanks!
Another thing that I noticed was that "this" instead of "these" when the pronoun was plural ---> This is them.
I think it's related to the fact native speakers say "This is Mary and Jenny" and not "These are Mary and Jenny". Interesting...

The native speaker's brain is probably thinking:
This ( [thing / group / pair / couple] that appears before [me / us] ) is ...

CJ