All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
In principle, no one can know the reality of anything that is a matter of the Unseen or of future events, except through the revelation of the Quran or the Sunnah.
In brief, Ya’jooj and Ma’jooj are descendants of 'Aadam (Adam) and were present at the time of Thul-Qarnayn. They used to attack their neighboring towns in the East where they spread corruption. They will also return at the end of time.
The Prophet stated that 'Eesaa (Jesus), the son of Maryam (Mary) will invoke Allaah against them and Allaah will kill them with worms and send birds to devour their corpses. [Muslim]
It was also authentically narrated that the Prophet mentioned that they are enormous in number. [Al-Bukhari and Muslim] In another Hadeeth, he mentioned that a small hole was opened in the barrier that holds back Ya'jooj and Ma'jooj. [Al-Bukhari and Muslim]
There are many further sayings reported by the Companions about their appearance and corruption but we cannot depend on any of them as evidence as to what they are.
We believe that this amount of information is sufficient and there is no need for further details. It is better to concern yourself with what serves the current affairs of Muslims.
And Allaah Knows best.