User blog comment:KaiserLos24/Yordle Lore = Corki Visual Upgrade?/@comment-5663427-20140327160927/@comment-620927-20140327201832

He didn't say Fizz is a Yordle; He said that Fizz and the Yordles share a common ancestor, which is very very likely considering the possibilities of evolution that have been displayed in real life. This was also something that Riot left up to interpretation, not having picked a particular side in the matter as to whether or not Fizz's people had any evolutionary connection to Yordles.

Besides, he's clearly 3 feet tall and as precocious every Yordle we've seen, so it's not hard to imagine that he's got Yordle blood in him.

Hell, maybe that's why Fizz is the way he is; Maybe his entire species isn't like him, and he's just half Yordle, half Merfolk or something.