Why does اژدها, a word of Persian origin, have the broken plural اژادر? And are there other examples of such pairs?

Answer 1

in fact, اژادر can be the broken plural of the word اژدر; and in ancient mythology, it was a very large snake that fire came out of its mouth; same was اژدها.

This type of plural for Farsi words is not correct , but sometimes, because of influence of Arabic, this might happen. Two other words فرمان and استاد come with broken plural inappropriately, and you may see them somewhere as فرامین and اساتید.

Answer 2

You are right, broken plural is not a normal plural in Farsi, it is normal in Arabic words. However, we have some of the words in Farsi that we use broken plural for them because of the sound(آوا) and they are more common than the plurals with(ها). But I would say most of the words that have broken plurals are Arabic words originally which are common in Farsi.