Robert Silverberg 
Thirteenth Immortal 

"Who was your father?" the mutant asked Dale Kesley. And try as he might, Kesley could not remember; his past was an utter blank. But he knew one thing – the answer to his life’s riddle lay in Antarctica, the once-frozen continent, now an earthly paradise surrounded by an impenetrable barrier.But how to get there? The only means of transportation were the spindly six-legged mutant horses. And it was suicide for Kesley to travel on the American continents. Two immortal dictators had set king-size rewards for his capture – dead or alive! But somewhere in the two continents there was someone who would help him, someone he had to find. The future of the world depended on his success.(First published 1956)