I think it's entirely possible that an undiscovered species of large mammal is out there somewhere. Just a couple of weeks ago they found 125,000 (yeah that's right one hundred and twenty five thousand)
lowland gorillas in the Congo that nobody knew were there. Granted this is not a new species but scientists not knowing they were out there certainly lends credence to the possiblity of smaller populations of other types of primates elsewhere. On the
island of Borneo alone over 400 new species of plants and animals have been discovered in the last 12 years.
Science is good but it doesn't have all the answers.