About 90%. The reason that the number is so high is that while people were discovering things via some method that resembled the scientific method, "scientist" was not an occupation. Science was pretty much invented by Sir Isaac newton and the number of scientists has exploded and increased exponentially since this time, hence the large percentage of scientists alive today.
For a full analysis of this question, including research on the number of PhD's granted in major countries since 1900, see Eric Gastfriend's work on the Future of Life Institute's blog. He concludes that around 80-90% is probably correct.
Scientists estimate that the species alive today make up less than 0.1% of those that have ever lived. See: http://www.sparknotes.com/biology/evolution/speciation/section3.rhtml
Scientists haven't identified all the species that are alive today because different species are emerging ("being made") all the time.
What percentage of people in the world in 1926 are still alive today
The blue whale, alive today.
Stephen hawkings and Jane Goodall
Some do say that unicorns are alive but scientists have no evidence of their existence.