What should I do if a black dog passes in front of me during my prayer in the third unit (rak'ah)?

Nothing interrupts the prayer, whether the worshipper uses a sutrah (barrier) or not, and whether the passerby is a man, a woman, or an animal. This is the view of the majority of scholars. It is sunnah for the worshipper to use a sutrah. If something passes behind it, it does no harm. If something passes between him and it, he should repel it. If he does not use a sutrah, his prayer is valid, but he should avoid places where people pass.