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I don't know of any genuine freaks of nature in the way described here, but that is definitely what they would be if there were such a thing, which of course there may well be - I'm not disputing that much.
However, there is also no shortage of videos appearing online which are very cleverly faked ones - and some, not so cleverly. Sometimes it's done through cutting & pasting repeated shots (no pun intended) together, so that it looks like one complete take. Other times it is done by way of fastening a concealed tube to the underside of the penis (or whichever part is out of camera shot), or even by using a silicone sheath, which can be virtually indistinguishable from the real thing, along which is injected liquid soap, cold cream, or some such fluid which can pass as real semen.
On other occasions the penis used in the cum shot is often a fake as well, with the semen also being faked. I recall such a fake penis being used with milk to represent semen in sex education while I was at school when it was being used to demonstrate how semen is released & conversely how a condom works as a barrier to prevent it from causing unwanted pregnancies. Although this was way back in the mid 70s I remember it still looked very realistic - so you can imagine how much further things have progressed these days, what with all the silicone technology now available, and if a professional porn company want to create such an illusion, it's not difficult to do so - there are even many clips where the faking is so badly done that it's funny, when you can see where the fake penis attaches to the real one, and when the repeat cuts don't 'quite' match up - often obviously taken from different sessions.