Clipped from Wikipedia and other fan sites.
Lost worlds : remote or near-inaccessible corners of our modern Earth where races and lifeforms still survive from prehistoric times have been a staple of fantastic literature since popular culture embraced the existence of dinosaurs. The term “dinosaur” was coined in 1842, by paleontologist Richard Owen, following the official recognition of the first discovered species, Iguanodon and megalosaurus, in the 1820s. As more of the remains of more incredible species were unearthed, it is hardly surprising that, along with the public’s fascination with prehistoric beasts, there was more than a little speculation that the living animals might still be alive somewhere. After all, there was still enough virgin territory at the close of the 19th century to make the possibility of fantastic monsters roaming the vastness of remote wilderness regions seem viable.
Edgar Rice Burroughs: had the distinction of being the creator of three separate lost worlds, Pellucidar, Pal-ul-don, and Caspak. Though Pellucidar and Caspak had their own separate series, Burroughs confined Pal-ul-don to a single novel, Tarzan the Terrible. Burroughs describes Pal-ul-don as a region “where every known species of bird and beast appeared to have sought refuge from the encroaching numbers of men since the first ape shed its hair and ceased to walk on its knuckles."
Unfortunately, Burroughs single novel did not allow him to develop Pal-ul-don further. Tarzan never returned to the lost land in any of the original novels, although he did revisit Pal-ul-don in many of the stories concocted by many of the comics writers over the years. The most notable of these writers seems to be Russ Manning, who took Burroughs’ hero to the lost land a number of times in the Sunday strips. Manning did indeed allow Tarzan to explore Pal-ul-don further, adding his own embellishments as to how the strange world worked. He also invented a Weiroo-like race of winged humanoids, who, like their Caspakian counterparts, were a race entirely of males, and who constituted an even greater threat to the women of Pal-ul-don than rampaging Tor-o-don bulls. It was Manning’s concept of Pal-ul-don existing in its own separate time-frame which is most notable, however. This invention may have been in part due to the fact that the existence of a hidden valley full of prehistoric beasts and races, remaining undiscovered in modern Africa, seemed less feasible in the later 1960s than when Burroughs wrote.
History of Pal-ul-don: When Jan goes missing Tarzan tracks her to a hidden valley of Pal-ul-don (Land of Men). Pal-ul-don is filled with dinosaurs, notably the savage Triceratops-like Gryfs, which unlike their prehistoric counterparts are predatory. The lost valley is also home to two different races of tailed human-looking creatures, the Ho-don (hairless and white skinned) and the Waz-don (hairy and black-skinned). Tarzan befriends Ta-den, a Ho-don warrior, and Om-at, the Waz-don chief of the tribe of Kor-ul-ja. In this new world he becomes a captive but so impresses his captors with his accomplishments and skills that they name him Tarzan-Jad-Guru (Tarzan the Terrible)!