No, it's derived from a virus, not an independent living organism. 8% of human DNA is derived from viruses that infected our ancestors and inserted themselves into our genome in order to replicate (which usually isn't passed on): https://theconversation.com/humans-are-8-virus-how-the-ancie...

I passed high school biology with a C, but I think the difference here is that two completely distinct independent organisms merged into one. The mammalian placenta seems to be more of an example of animals appropriating genetic code from viruses, which I suspect is something far more common in evolution.