In your last scenario it would not very likely be the case of a newly evolved snake within 200 years, due to the inadequacy of the elapsed time frame for such an event. The gradualism for speciation is not likely on that short of a time scale. But if you look at other examples your question takes on a more plausible nature. Domesticated farm animals (or pets), I think is your "best bet" as far as finding an example of initial speciation steps in vertebrates being observed my man. Also wild animals adapting to fit into a modern man-made context might be an area to focus on, like the modern foxes, coyotes, racoons, falcons, pigeons, rats and insects in cities or crop fields.
As far as humans having missed vertebrate sized animals for thousands of years, or otherwise long enough for them to have gone unnoticed as a cladogenesis event took place, highly unlikely.
At the end of the day, I think you're asking how old are the species we are living beside here.
Edited by CosmicChimp, : more examples.