Goblins (aka Swamp Goblins) and Hobgoblins arrived on Oceana in 300 BCE. They landed on the same island as the Egyptians. War ensued fairly quickly. The war never ended due to the size and geography of the island.
As the Goblins and Hobgoblins were forced from the prime agricultural land, a significant number of the various clans moved to other islands. As the humans, and other allied races continued to expand, the goblins and similar "monster" races were forced further and further west. Many were killed in the fighting, and many more were enslaved.