How do I pay zakat on the deferred Mahr (dowry) that I have not yet received from my husband, given that more than a year has passed, and I was unaware of the obligation of zakat upon it? And do I pay its zakat from my own money?

If a year has passed on the deferred dowry (mu'akhkhar al-sadaq) and the husband is able to pay it, then zakat is due on it if it reaches the nisab (threshold). Jurists have differed on whether zakat is deferred until the wife receives her dowry and then pays zakat for the past years (the view of the Hanbalis), or if she pays zakat at the end of each hawl (lunar year) even if she has not received it (the view of the Shafi'is). However, if the husband is in financial difficulty (ma'sir) or procrastinating (mumāṭil), then no zakat is due on it until it is received, and at that point, it is subject to zakat for one year according to some scholars.