Kemokemo is a creature introduced in episode 48 (223 overall), in which Natsu and Happy hatch it from an egg that fell from the sky, and give it its name.

While Kemokemo steadily grows without needing to eat or drink anything, Natsu asks everyone he can for answers as to what Kemokemo is, but the search comes up empty. Kemokemo himself questions his existence, but finds contentment in his happiness with Natsu, even protecting him from perceived threats.

It is revealed that Kemokemo is a protector nature god of an island that periodically rises from the sea and releases a virus in waves.

As explained by Lucy, Kemokemo was the one who saved the ancient civilization who lived on the mysterious island that releases the virus, by using his powers to purify the whole place, and still continues to do what he does, even today.
(Source: Fairy Tail Wikia)