Ninki Nanka

A dragon like creature is believed to exist in the swamps of Gambia. 50 meters long by 1 meter wide, with a face resembing a chinese dragon, it is said to inflict death on eye witnesses. A team of dragon hunters from UK is currently launching an expedition in search of it.

This is different from discovering news species of mammals. For undiscovered animals of this size, I'll put my money in the ocean's fathoms.