Homo floresiensis - the most recent living human relative
Why did they go extinct? I'm betting when modern humans encountered them on their Indonesian island those people thought here was something that might be good to eat and very easy to hunt and kill. It's too bad. If they still lived today I can't imagine anything more interesting.
For more information please see Homo floresiensis was a one metre tall, human-like creature living and using tools in Indonesia just 18 000 years ago and was a distinct species, not just a malformed modern human. The so-called hobbit had wrist bones almost identical to those found in early hominins and modern chimpanzees, and so must have diverged from the human lineage well before modern humans and Neanderthals arose.