"Dieser Hund ist mein."
why is "this" in female form(Dieser), but "mine" in male form (Mein)? are there any rules or just random ??
'Dieser' is masculine, since it's 'der Hund'. 'Diese' is feminine. 'mein' is nominative.
I'm not sure if "mein" is used correctly here. It sounds quirky at best.
Therefore the whole sentence does, in my opinion. Would you actually say that? Starting from the english sentence, I would've suggested to say: Dieser Hund gehört mir. Maybe if you wanted to keep it simple: 'Das ist mein Hund'.