according to hindus, human birth is rare. its a long process of evolution to be born as human being. you have to travel 84 lakhs wombs (yonis) to take birth as human. so it is considered a good fortune to be born as a human being. it is suggested that one should use this rare opportunity to realize your true nature.
when they mention about 84 lakhs wombs (yonis) then it does not mean that after this human birth you will have to again take birth in 84 lakhs wombs to get human birth again. it just means that human birth is the most evolved womb and you have spend time in 84 lakhs wombs to evolve yourself.
so the question is once you have evolved in consciousness and taken birth as a human then is it possible that in your next life you will be born in animal kingdom?
according to hindu books it is possible as there are many stories in hindu scriptures where such things have happened. there are also stories where celestial beings have fallen down and taken birth as human being because of some curse or desire. if at the moment of your death you are too much attached to your pet or your attention is fixed totally at any animal or bird then you might be reborn as them. the moment of death determines your next birth. how you die or leave the body is very important.
hindus try their best that when their loved one is dying then they don’t have any unfulfilled desire (trishna) so that their next birth is higher. dying with lot of desires unfulfilled is not considered good. many spiritual people also make sure that in last months of their life, they become detached with their loved ones. they go totally into their spiritual practice and let go of the matter related with family. all this is done to elevate their consciousness and to become dis-identify with the family. it is done to prepare themselves for the ultimate journey of death. so in hinduism lot of importance is given to the mental and emotional state of the dying person.
coming back to the topic, only osho (acharya rajneesh) refute this theory of hindus. osho said that he has taken thousands of people in past life via hypnosis. but he didn’t found one case where a human has regressed back to animal kingdom.
according to me, one can be born in animal kingdom. because if your thoughts, actions and karma are that of an animal then automatically you are regressing down. so i stick with the theory of hindus that you can be temporarily (like one lifetime) born into an animal kingdom. it can happen if at the moment of death you are totally worried about your pets or your total attention/energy is going towards them. this is a rare case. because most people are not so total with their energies and most of the time we are thinking about money, family or other things while dying. very rarely someone think about his pet while dying and then also the thinking has to be very strong and total to be born in animal kingdom. so it is possible but it is a rarest of rare case.