"This dog is mine."

Translation: Dieser Hund ist meiner.

[JAM_PT]

why "meiner" ?

[Bharad.kv]

https://en.wiktionary.org/wiki/mein#German
When used as a noun, the nominative masculine takes the form meiner, and the nominative/accusative neuter takes the form meines or meins.

[Bharad.kv]

my question and sakasiru's response:

> This is not genitive, is it? What is meiner here? Possessive determiner?
Yes.

"meiner" can actually be different things, but here, it's an inflected form of the possessive pronoun "mein".
It's nominative though, you could say "dieser Hund ist der meine instead.

> I came across an entry in Wiktionary that says meiner could also act as a noun sometimes. Is meiner a noun here? Is meiner nominative predicate?

It is a pronoun (personal, possessive or reflexive), and since pronouns are stand-ins for nouns, it surely can act as one. I'm not sure whether you actually sort it as a nominative predicate, but I don't think it would be wrong.

[Bharad.kv]

Also: If the noun is omitted and the possessive stands alone as its replacement, it must still retain the gender, number, and case of the noun. In this form, it is no longer a modifier, but a pronoun.

„Ist das Ihr Wagen?“ „ Ja, das ist meiner.“ (mein Wagen)
“Is that your car?” “Yes, that’s mine.”