When somebody says that a new species has been discovered, it's easy to get the impression that this is an animal nobody has ever seen before. But that's usually not exactly what scientists mean.
Take the lesula (or Cercopithecus lomamiensis), an African monkey whose "discovery" is making headlines this week. While it does seem to be true that this particular species hasn't been previously named and documented in the scientific literature, the scientists who wrote about the lesula were not the first people to encounter one. What's more, lesula do not represent a species totally removed from animals we already knew about. Here's Mongabay's Jeremy Hance:
"There are monkeys out there between the three rivers that no one recognizes. They are not in our field guides," Terese Hart wrote tantalizingly in a blog post in 2008. "We've sent photos to the most renown of African Primatologists. Result: a lot of raised eyebrows. And the more we find out the higher our eyebrows go."
One of these monkeys was the lesula (Cercopithecus lomamiensis). John Hart first came across the new species in June 2007 when he and a field team were shown a captive baby lesula, kept as a pet by the local school director's daughter in the remote village of Opala. The next step was locating the species in the wild.
...the lesula is apart of the Cercopithecini family, which are commonly referred to as guenons. It's most similar to the owl-faced monkey (Cercopithecus hamlyni), which is also found in the region. But the lesula sports a lighter coat and has unique calls. Genetic testing, furthermore, proves the species are distinct from each other and have likely been separated for a few million years, probably by impassable rivers.
I don't mean to downplay this find. It's pretty awesome for the Harts to be the first scientists to write about this species in a peer reviewed research paper. But I think there's a bit of a common mis-understanding between what scientists mean when they say they've discovered a new animal, and what the public often thinks they mean.
Hopefully, this will give you a better idea of what's going on.