An extinct species of lemur, known from fossils on the island of Madagascar. It is the largest primate known to ever have lived. The skulls of adult megaladapa are almost two feet long -- the animal as a whole was the size of a small cow.
Megaladapis came in three species, M. madagascariensis, M. edwardsi, and M. grandidieri. All three are also called the "Koala Lemurs".
Remarkably, radio-carbon dating shows that these creatures lived until quite recently -- certainly as recently as 1000 years ago, and perhaps much more recently. Humans migrated to Madagascar only 1500 years ago, which provides circumstantial evidence that we wiped the things out.
Madagascar is isolated and had no primate species other than lemurs before the arrival of man. This gave the lemurs a chance to diversify into evolutionary niches unavailable to them elsewhere, and they are found in many strange forms there, from very small to (now extinct) extremely large.
An 18th century French explorer named Francois de Flacourt visited Madagascar in 1758 and described a creature called the "tre-tre-tre-tre" by locals. Supposedly this animal was the size of a two-year-old calf and had a human-like face with monkey-like hands and feet. Some paleontologists suspect that this was a megaladapis, and that the creature escaped the attention of western science by only decades.
Richard Milner, Encyclopedia of Evolution, 1990
Walker's Mammals of the World Online (http://www.press.jhu.edu/books/walker/primates.megaladapidae.megaladapis.html)