Well, i'm going to tell you that it is POSSIBLE. However it is HIGHLY unlikely. First off, it sounds like strait out BS. My bullshit-dar is going off like a chicken with its head cut off. But it is possible. I don't see much reason for them to do that however, which furthers the unlikeliness. If they are just looking to put agents on the planet, it is reasonable to believe that a race of beings sufficiently advanced to mate humans and non-human creatures together, would also be sufficiently advanced to put pure alien agents that are disguised somehow into the populace. If they were looking to influence human evolution, then it would probably be easier to just directly genetically alter humans.
It is very difficult to mate two species together, the body will spontaneously abort anything that will have serious genetic problems, or it just wont fertilize, it is the rare and unlucky ones that make it to life with horrible genetic disorders. So mating a human to a non-human would require extensive if not COMPLETE understanding of human genetics to create a being that when impregnating a human would not cause a spontaneous abortion muchless make it to maturity and be fertile with other humans.
So ignoring that there is practically no reason to do that except curiosity and general entertainment, it is possible, but so incredibly unlikely that this is probably just total BS.
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And as always, these are simply my worthless opinions.
My body is a channel that translates energy from the universe into happiness.
I either express information, or consume it. I am debating which to do right now.