Is it possible to realize Dzogchen in these non-human realms or in non-human body because Mind Direct Transmission (Tib. Gyalwa Gong Gyü) lineage or Sign Transmission of the Vidyadharas (Tib. Rigdzin Da Gyü) lineage can be somehow "delivered" there? Is it possible to have Oral Transmission (Tib. Gangzak Nyen Gyü) there also?
I think it is important and beneficial to know it because one can choose what to practice (Dzogchen or Highest Yoga Tantras). It is said the precious human body is very rare to obtain once more and if Dzogchen is possible also in non-human bodies one can invest for one's own safer future too
I think those nagas, daka/dakini and deva/devi who attain enlightenment through rig sum gongpo or maha siddha's teaching are in category of gangzak byen gyu too. Those teachers spoke, and they listened.
For knowing the nature of mind, I don't think other kinds of beings would have huge problem. The problem is the path people choose to relise the nature of mind. Most of HYT use a method that require human body, and other methods are not mentioned in those tantra, therefore the conclusion would be you got to have a human body.
The point here shouldn't be if one should choose HYT just because he happens to have a human body. It should be that you have a human body, so you have two choices, and you now check carefully which one is better for you. We got to know that the difference in sense of human body or not, is just about if you can achieve buddhahood before you die. Only when considering the possiblity of attaining Buddhahood within one life, then it's meaningful. How many people attained full Buddhahood in life time in history? Not many.
Also, people got to think the difficulty of paths. Even if you got a human body, doesn't mean that you can pratice those methods which especially designed for human body. There are very high requirements.
I think for common people like me, who don't have time to do years of retreats and to leave families and practice alone, human body is precious mainly because the reasons described in Sutrayana teaching. But I am happy still that I have the chance to practice some of HYT methods, just probably I couldn't grasp the chance as well as great masters.