The Tendas are possibly a race that comes from where Giygas hides at the end of the game. This theory is as simple as can be. When in the Lost Underworld we notice two caves. One leading to the Hot Springs where you fight Carbon and Diamond Dog. And one leading to an all too familiar resting point... In fact when you enter this cave you are entering the resting place you goto before you go to fight Giygas... But here's another thing... This cave is identical to the "Cave of the Past," it just has grass and is a little more jagged... This is because it is the "Cave of the Past." Hmm maybe you might have been able to climb up that ridged cliff and walked to where Giygas was in the past, in the future... I know this is hard to explain, but suppose the Tendas lived in the "Cave of the Past" until Giygas came, he pushed them out, and thats why they inhabit the Lost Underworld.