I’ve read that various Hindu Gods (i.e. Ganesha) are actually given 'life' and an astral reality due to the tremendous energy generated by the masses' devotion to that form. Could this also be true of other unexplainable phenomena, i.e. the Loch Ness Monster or Bigfoot, where people’s intense interest actually 'create' an astral reality, such that those forms can 'cross over' sporadically onto the physical plane so they are witnessed by folks?
I remember Swami Kriyananda being asked this same question, about the “reality” of the Hindu gods and goddesses. He gave essentially the same answer you have mentioned. That is, the great devotion generated by masses of worshipers can cause the entity to take a “real” form, from time to time.
As for other unexplained phenomena such as the Loch Ness monster or Bigfoot, what you are mentioning about their having an astral reality seems quite possible to me, though I don’t remember and cannot find any specific reference to these two (or anything similar) in the teachings of Paramhansa Yogananda or Swami Kriyananda.
There are many, many things in our world, which could come under the heading of “unexplained phenomena.” There are probably a lot more “crossover” situations between the material world and the astral world than most people can even begin to imagine.